The Inverted L Antenna and NVIS

In this article LodeRunner explains what an Inverted-L antenna is, how it works, and why you might strongly consider building one for use on the lower bands. While explaining its use with a high degree of technical information, he’s written it in a manner that’s easy to follow and digest. The diagram data is sourced from EZ-NEC, which a link to the software is provided in the sidebar.

The Inverted-L Antenna and NVIS

An “Inverted-L” antenna is basically a wire antenna, typically ¼ to ½ wavelength long on the band it is designed for. The Inverted-L antenna is a common antenna for the 160 meter and 80 meter amateur bands, where typical ¼ wave verticals are impractically tall for most amateurs.

In the Inverted-L configuration, the first portion of the wire rises vertically from the feedpoint, and at some height is bent roughly 90 degrees, and then extends horizontally to the unterminated end. The feedpoint is very close to ground level (typically not more than 3 feet above ground), and the antenna is worked against a Ground consisting of one or more ground-rods, and/or a counterpoise consisting of one or more radial wires – which may be buried, laid directly on the ground, or suspended above the ground at some low height.

Because the input impedance of a typical Inverted-L antenna is low, and the feedpoint is at or very close to ground level, where ground losses are substantial, it is very important to establish a good ground to work the antenna against.

Most of the amateur literature regarding the Inverted-L antenna is focused on optimizing performance of the Inverted-L antenna for “DX” operation – that is, high efficiency in radiating its energy in a pattern that is low in elevation (low Take-off Angle, or ToA) – typically below 30 degrees relative to the horizon – which maximizes the distance to which communications may be achieved. Effective NVIS communications, on the other hand, require an antenna which is optimized to produce a pattern where the majority of the radiated energy has a high ToA pattern – ideally between 60 and 90 degrees – to provide reliable communications from zero to several hundred miles.

Inverted-L1
Fig. 1: DX dipole pattern on 80 Meters

First, lets take a look at the difference between a good “DX Antenna” pattern vs. a good “NVIS Antenna” pattern –

Figure 1 is a diagram of a dipole optimized for DX communications; Figure 2 represents the exact same dipole, but the height has been lowered by approximately 1/3 wavelength to optimize the antenna pattern for NVIS communications.

Inverted-L2
Fig. 2: NVIS Dipole Pattern on 80 Meters

These diagrams make it easy to see the difference – first, the green line in each diagram shows the direction of maximum radiated energy – the Take-off Angle, which is commonly abbreviated “ToA”. For the DX dipole, the ToA is at 30 deg above the horizon. For the NVIS dipole the ToA is 90 deg, i.e. straight upward. In the above figures, it is the signal energy in the area between the two diagonal lines which produces NVIS propagation. The purple lines show the angles which are called the “Half Power” points. In simple terms, these are the ToAs where the signal strength drops to half of what it is at the best ToA of the antenna’s pattern. The angle between the two purple lines is called the “Beamwidth” of the primary lobe. The beamwidth of the NVIS dipole encompasses the entire NVIS “Sweet Spot” – which is roughly between 60 and 90 deg. ToA .

These pattern plots show us that NVIS and DX communications are essentially opposite objectives, so designs which optimize the antenna for DX will produce very poor performance for NVIS, and vice versa. Because I have not seen elsewhere any advice on optimizing an Inverted-L antenna for NVIS on the 160 and 80 meter bands, and because I believe that effective NVIS communications on these bands is essential for preparedness communications, I have prepared this article as a practical guide to achieving these objectives.

The objectives which will be satisfied are –

  • High efficiency NVIS radiation pattern for both 160 and 80 Meters

  • Feed impedance directly compatible with typical auto-tuners

  • Minimized Ground losses, maximized antenna efficiency

  • Build a simple matching device which provides an efficient, low-loss match to the feedline

  • Only a VSWR meter is required to adjust the antenna and matching unit for best performance

To meet these criteria, the antenna wire will need to be between 175 and 215 feet in length, and the horizontal section must have a height of 50 to 65 feet at each end. This means that the horizontal span of the antenna wire will be between 110 feet and 165 feet. A small amount of slope [ +/- 10 feet ] in the horizontal section is not a problem, and some sag in the middle of the horizontal wire unavoidable, but no part of the horizontal wire should be below 40 feet Above Ground Level (AGL) or above 65 feet AGL for optimized NVIS performance.

t’s important to note here that, even if antenna supports of greater than 65 feet are available, the antenna should not be hung at a height greater than 65 feet, unless the length is also increased (which will increase the challenge of matching the antenna to the feedline). If we hang a 200 foot wire at a height above 65 feet, then the Current Loop (point of maximum radiation) will be in the vertical section of the wire, and the NVIS radiation pattern will be substantially degraded (it will become a good DX antenna, but that’s not what we want in this case).

The antenna wire should not be closer to any non-conducting materials (such as tree branches) than 6 feet, and ideally the minimum separation from branches/support poles/building roofs/etc. should be 20 feet or more, in order to minimize the RF losses in the near-field of the antenna.

Separation from metallic objects should be at least as far from the wire as the antenna is above ground, i.e. 50 feet or more – this includes flag poles, utility poles (they have a ground conductor running down their entire length), metal roofing/siding of buildings, etc. This is particularly important because metallic materials in the near-field of the antenna may have substantial effects on its radiation pattern and efficiency.

DEFINITELY ENSURE THAT NO PART OF THE ANTENNA CAN COME IN CONTACT WITH ANY UTILITY LINE, EVEN IF ONE OF THE SUPPORT ROPES FAIL WHILE SUBJECT TO STRONG WINDS. YOUR LIFE, AND THE CONTINUING OPERABILITY OF YOUR EQUIPMENT DEPEND ON MEETING THIS REQUIREMENT.

First, let’s take a look at the typical (¼ wavelength) Inverted-L antenna’s pattern, and why it favors DX over NVIS communications –

Inverted-L3
Fig. 3: Elevation Pattern of a 1/4 Wavelength Inverted L Antenna

Looking at this elevation plot and the associated data, we see that the maximum gain occurs at 30 deg., and that this gain is roughly equivalent to a dipole above typical earth, i.e. ~3dBi (although this antenna, unlike the dipole, produces mostly vertically polarized energy).

We can see that the radiated energy in the NVIS sweet spot from 60 to 90 deg. is down 3dB to 5dB from the peak. Remember that -3dB = 50% power, a.k.a. The “Half Power Point”, so the NVIS signals are at least 50% weaker than those at the 30 deg ToA peak. This is obviously a DX antenna on 160 Meters, and would be very difficult to match on 80 Meters, so I won’t spend the time to go through the 80M plots.

Another important consideration with Inverted-L antenna is their losses. With a ¼ wavelength Inverted-L, the point of maximum radiation is right at the feedpoint, just above the earth. With typical soil conditions, a large portion of the signal radiated into the ground is absorbed, lost.

Inverted-L4
Fig. 4: Point of Maximum Radiation and Consequent Losses

In the above diagram of a ¼ wavelength Inverted-L antenna, the red circle indicates the area where the strongest field-strengths occur. Notice that essentially half of our radiated energy will interact closely with the ground. Depending on soil characteristics and the quality of the radial field under the antenna, somewhere between 30% and 70% of this energy will be lost.

Now, lets look at what happens when we change the antenna wire from ¼ wavelength to 3/8 wavelength, again at 1850Khz –

inverted-l5.png
Fig. 5: Elevation Pattern of 3/8 Wavelength Inverted L Antenna

Pictured right is an Elevation Plot of an Inverted-L antenna which is 50 feet in height, and has a horizontal span of 150 feet – so the radiating element is 200 feet in length – this is roughly 3/8 wavelength at 1850Khz.

This pattern is much better for NVIS: the ToA for peak power is at 80 deg, and at no point in the 60 – 90 deg “NVIS Sweet Spot” is the signal strength down more than 1dB from that peak. You should also notice that the Max Gain is 5.69dBi. This is 2.6dB more gain than the ¼ wavelength Inverted-L, and the Beamwidth of this antenna includes the entire NVIS Sweet Spot. In round numbers, this antenna puts about four times as much RF energy into the NVIS Sweet Spot as the typical Inverted-L designed for DX work.

With regard to losses, we can see that moving the point of maximum radiation up into the horizontal wire has substantially reduced the portion of the radiating field which must interact closely with the earth. In Figure 6 below, the small blue circle indicates the (approximate) point where maximum radiation occurs on 160M. Increasing the height of the point of maximum radiation (in this case from 0 feet to 50 feet) reduces earth losses by approximately 1 to 3dB – again, depending on soil conditions and the quality of the antenna’s ground and/or counterpoise.

inverted-l6.png
Fig. 6: Elevating the Point of Maximum Radiation

Another effect of moving the point of maximum radiation up into the horizontal section is this – the feedpoint has a much higher [real] resistance. Since the efficiency of an antenna is determined by the ratio of Rrad (radiation resistance) to the sum of Rrad + Rloss {Efficiency = Rrad / Rrad + Rloss}, we have two ways to improve the efficiency – increase Rrad and/or decrease Rloss. In the case of an Inverted-L antenna, it is much easier to increase Rrad than decrease Rloss,and increasing the efficiency of the antenna accounts for a good portion of the increased gain of the 3/8 wavelength versus the ¼ wavelength Inverted-L.

So how does this 50ft/150ft antenna perform on 80 meters? Lets take a look –

inverted-l7
Fig. 7: 80M Elevation Plot, in line with horizontal wire, e.g. North-South

I’ve included both elevation plots (Fig. 7 & Fig. 8) because I want to make obvious that the orientation of the horizontal wire is very relevant to the coverage provided by the antenna on 80M. So if you desire broader coverage East-to-West, then the antenna wire should be hung North-South, just as you would for a dipole.

Notice in Figure 8 that there is a vertically polarized radiation pattern from the antenna (in red) which will provide some East-West coverage beyond NVIS range when band conditions are favorable.

inverted-l8
Fig. 8: 80M Elevation Plot, perpendicular to horizontal wire, e.g. East-West

All of the preceding assumes that the antenna is worked against a “good” ground. At a very minimum, this antenna requires three radials of at least 100 feet in length, and the efficiency and pattern will be much better when the antenna is worked against a set of 6 or more radials (ideally) of at least 150 feet in length each.

Using the minimum of 3 radials, the layout should look something like Figure 9, where the red line indicates the antenna wire, and the black lines represent the ground radials. The center radial is directly below the antenna wire, and the two outer wires are “fanned out” to run parallel to the center wire at a distance of 15~25 feet to either side. If the three radials can be extended to the same length as the horizontal section of the antenna wire, this will improve the antenna’s efficiency and VSWR at best match.

inverted-l9.png
Fig. 9

inverted-l10.png

A better arrangement of ground radials is shown in Figure 10, above. The three radials going to the right of the antenna feed point are the most important, and the optimal length for these is ~150 feet each. The radial going to the left of the feedpoint is least essential, and can be bent, shortened, or omitted if space constraints require. The outer half of the radials going vertically in Figure 10 can be bent to suit the available space without substantial effect on the efficiency or pattern of the antenna; if these radials are shortened, this may affect the efficiency and/or matching requirements, but not to such a degree as to make the design of the antenna matching unit described below unworkable.

At least one ground rod should be installed at the feedpoint of the antenna as a lightening ground, and a suitable surge protector/lightening arrestor device installed at the feedpoint.

~~~

Since construction of the 3/8 wavelength Inverted-L antenna was thoroughly covered in a previous post by JohnnyMac, I’ll move on to describe how to obtain a simple and high efficiency match from this antenna to your feedline.

On 160 Meters, a simple series capacitance will suffice to resonate the antenna on 160 Meters. Depending upon the details of wire length, height, and soil conditions, a total capacitance of 80pF to 180pF will resonate the antenna on any desired frequency within the 160 Meter band. The VSWR curve will look like this –

At the bottom of the 160M band, a capacitance of 145pF (0-j600 ohms) gives us this:inverted-l11

While not perfect, this is a very workable match for the auto-tuners built into many modern transceivers, as well as external tuners such as produced by LDG and other companies.

In the top half of the band, 102pF (0-j800 ohms) obtains a nearly perfect match to a 50 ohm coaxial feeder all by itself:inverted-l12.png

Looking at possible matching solutions on 80 Meters – again, a simple capacitance of 560pF (0-j75 ohms) gives us a very usable mid-band VSWR plot as follows –inverted-l13.png

Towards the bottom of 80M, no capacitance at all is required for an excellent match. This is the point at which the antenna wire is ¾ wavelength long. This VSWR plot is with a direct connection between the feedline and the antenna wire –inverted-l14

What this tells us is that, to obtain a good match at the bottom of 80M with a 200 foot (50’v/150’h) radiating element, an inductive element (coil) will be needed at the feedpoint to match the antenna, which would complicate our matching arrangements – but should be within the ‘reach’ of most commercial matching units.

However, if we increase the length of the wire a bit, we can shift all the needed tuning reactances into the negative (capacitive) range across both bands, and keep our matching system simple and efficient for both 160M and 80M.

If we extend the wire another 10 feet horizontally (50’v/160’h) then 93pF (0-j900 ohms) obtains a near-perfect match at 1900Khz. Fig. 15.png

And 121pF (0-j725 ohms) will give us a very workable match at the bottom of 160M –Fig. 16.png

Adding 10 feet to the horizontal section shifts our 80M matching points as follows —

At 3600Khz, 590pF (0-j75 ohms) obtains a near-perfect match which looks like this:Fig. 17.png

Obtaining a 1:1 VSWR match at 3500Khz would require over 1000pF of capacitance (0-j40 ohms) so the lowest frequency with a ~2:1 or better VSWR match using this method will be approximately 3550Khz. This limitation only applies if you are building the simple capacitive matching unit described below – with a commercial tuner a 1:1 VSWR can be achieved anywhere in the 80 Meter band.

In the middle of the 80M band, 236pF (0-j180 ohms) obtains this very good VSWR curve –Fig. 18.png

And at the top of 80M , 115pF (0-j350 ohms) obtains a very workable match, also –Fig. 19.png

With all this data, we can confidently conclude that an Inverted-L antenna of 200~215 feet in total wire length, with a vertical portion 50 to 65 feet in height, can be effectively matched to a 50 Ohm coaxial feeder on 160M with nothing more than an adjustable capacitance in series between the feedline and the beginning of the antenna wire. Assuming we can install the antenna with a total length of 210~215 feet, then a series capacitance is all that is required to resonate the antenna across most of the 80 Meter band, also.

If your primary interest is towards the bottom of both bands (CW and Data segment) then a total radiator length of 210~215 feet is optimal (depending on soil conditions); if your interests incline more towards the middle/upper portions of the band for ‘phone operation, then 200~205 feet will suffice.

Assuming that you are able to place a good antenna tuner right at the base of this antenna, then no series capacitors (outside the tuner itself) should be required to obtain a good match across most of both bands, and you are ready to get on the air.

If you wish to “roll your own” tuner, either as a stand-alone, or to bring the VSWR to a workable value right at the feedpoint (and then finish matching with an auto-tuner built into your rig, or located away from the feedpoint of the antenna) then read on.

Assuming a 210 foot total wire length (50’v / 160’h) over a good counterpoise for all of the below calculations, then:

At 1900Khz we simply adjust the series capacitor for the best match, which will be very near the calculated value of 93pF(some value between 90 and 100pF will obtain the best match), and a variable capacitor covering the range of 85 to 125pF will provide a match of better than 1.5:1 across the entire 160M band. If this is not the case, then the length of the antenna wire should be adjusted to achieve the best match at 1900Khz with 90~95pF of capacitance. This will have to be determined experimentally once the antenna wire is installed in it’s final location. If you can get a “perfect” 1.0:1 VSWR at 1900Khz with a capacitor setting very near 93pF, then the antenna is optimized for 160M, and you are set to operate across all of 160 meters – only the variable capacitor will need adjustment.

To obtain a 1:1 match at the bottom of 80M (~3600Khz) only the series capacitance is required, and this will be very close to 590pF. Since this is a larger value than commonly available in a transmitter-rated variable capacitor, a combination of a fixed capacitor (e.g. 250pF, 400pF, or 500pF) and the variable cap in parallel can easily be used.

To cover all of 160M, and the majority of 80M the wiring of the “match box” looks like this –

Fig. 20.png
Fig. 10: Simple Tuner for 160 and 80 Meter Inverted L of 210~215 Feet total length

For a transmitter of up to 150 watts, suitable components are as follows:

  • C*Tune needs to be a variable capacitor with a Max value of at least 130pF and be rated for at least 500 volts RF. 250PF is the ideal Max. value for this variable capacitor. 250pF and 150pF transmitting variable caps are a very common find at hamfests, and can often be purchased for as little as $10~$20 dollars. Ideally you want a 250pF unit, to give you the broadest possible tuning range on 80 Meters.
  • C*Fixed is a typical “doorknob” transmitting capacitor rated for at least 500 volts RF, and having a value of 500pF (if your C*Tune is a 150pF unit) or 400pF (if your C*Tune is a 250pF unit). Two fixed capacitors of lesser value may be used in parallel, e.g. two 200pF units to obtain 400pF total, or two 250pF units to obtain 500pF total. The objective here is to obtain a maximum combine value of 600pF ~ 650pF, so that there will be sufficient tuning range to reach the bottom of 80 Meters, and roughly 2/3 of the total capacitance should be in the Fixed capacitance so that adjustment of C*Tune on 160 Meters will not be too difficult (fine) to find the best match easily.

SW1 is a DPDT manual switch or relay rated for 250VAC/5Amps or more. If using a relay, the coil voltage can be whatever is available and convenient – relays with a 12VDC coil are common, and can be powered from the same power supply as your transceiver. 16Ga. or 18Ga. “Zip Cord”, or “Thermostat Wire” are inexpensive, and can be used to carry the switching power from the control position to the matching unit at the feedpoint of the antenna. The entire match-box assembly should be built in a good weatherproof enclosure.

If you plan to operate at power levels >150 and <=1000 watts, then the voltage ratings of the capacitors should be higher – 2000 Volts for up to 1000 watts of transmitter power, and SW1 must be rated for at least at least 1000VAC/10Amps per set of contacts. Otherwise, flashover of the relay and/or capcitors will occur and your transmitter may be damaged.

Other configurations of antenna wire and/or matching unit are certainly able to produce a good NVIS pattern on 160 and 80 Meters, but for all possible configurations the key elements for optimized NVIS performance of the antenna are:

  1. The horizontal portion of the wire should be no more than 65 feet AGL to maintain an NVIS radiation pattern on both 160 and 80 Meters;
  2. The Current Loop (where the majority of signal is radiated from) must be in the horizontal section of the wire on both bands, and
  3. You must work the Inverted-L antenna against a good ground or efficiency will be reduced and matching will be more difficult to achieve

I hope this article has given you all the info you need to put a strong NVIS signal on the air. I will answer questions posed in the comments section.

73, LodeRunner

~~~References and Annex Materials~~~
  • ARRL Antenna Compendium Vol. 7 (2002) ISBN: 0-87259-860-8, “Horizontally Extended Inverted-L and Flattop Vertical Antennas”, Pp. 17-21
  • https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html – useful for calculating matching networks for homebrew -at-feedpoint matchboxes
  • https://www.everythingrf.com/rf-calculators/whip-antenna-calculator – useful for calculating STARTING POINT resonant lengths and matching components for Inverted-L as well as vertical “whip” antennas.  Treat this as a “quick and dirty” way to get into the ballpark, then use EZNEC to work out the fine details.
  • http://toroids.info/  Not just for calculating turns on toroids.  This tool also allows you to calculate inductance and capacitance needed to give you a particular reactance at a given frequency, or vice-versa.

EZ-NEC file data for 210ft antenna:

  • 50ft height / 150 ft span. 200 feet total wire length.
  • 1850Khz – 3/8 wavelength Inverted L at this frequency
  • Max current is in Wire2/Segment2 @1.44 times the feed current.
  • VSWR@1850Khz = 1.39:1 for a 50 ohm Zfeed / 1.44:1 for 25 ohm Zfeed
    Zfeed is 35.9 -j0.73 ohms
  • For full-band coverage of 160M with an auto-tuner, a 150pF capacitor placed in series with the feedpoint is more than adequate.
  • 80M coverage —
  • 0+j135 ohms (6.14uH coil) resonates this antenna at 3500Khz
  • 0-j200 ohms (200pF) resonates this antenna at 3950Khz.
  • Therefore, full 80M band coverage is well within the range of any competent auto-tuner placed at the feedpoint. A relay should be employed to shunt across the 150pF capacitor for 80/75 Meter coverage.
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14 thoughts on “The Inverted L Antenna and NVIS

  1. Chris

    What an excellent article! For those with the space, and vertical support, this is an excellent way to get on the “top band.” LodeRunner has given us a thorough discussion of the inverted L and NVIS operation.

    Liked by 1 person

    1. LodeRunner

      Gary, my intention was exactly this – to get people over the hump to actually building such an antenna – the “DO IT” that turns knowledge into experience.

      And NC Scout and I are here to answer questions, if folks run into difficulties.

      Liked by 1 person

  2. Pingback: Brushbeater: The Inverted L Antenna And NVIS | Western Rifle Shooters Association

  3. Quietus

    Hey, I’ll bite. My first QSO in ’12 was on 160m with a 75′ end fed. There’s been some antenna improvements through the years on my end. Currently am using a droopy dipole cut for 3910, feed point at about 39′ and ends at about 15′ above ground. This antenna works like gangbusters most of the time on 80 meters.

    But I’m feeling some pain currently, when I have to be relayed in to net control 150 miles away. The band is changing, it is what it is and what it may be for some time. I can deal with loss of local comms on 80m, while enjoying the band going long and getting into stations a thousand miles away.

    Meanwhile I recognize that a new antenna needs to be built, for 160m. My tuner won’t tune my house dipole for 160m.

    I am thinking along the lines of an end-fed at a short elevation from the house, running 175′ of wire to a 40′ mast, with a 9:1 unun and about 50′ of counterpoise wire run out at 90 degrees to the radiator. Thoughts or attacks on this proposal?
    Advice is appreciated.

    Right now my truck is chained up on National Forest lands, camping. Have made a mast for the Slim Jim co-axed to a Wouxon HT. House radio is Icom IC-7200, works fine for me.

    Liked by 1 person

    1. Quietus:

      You may have better results with a longer counterpoise; wire lying on the ground looks longer than if it’s way up in the air, but 50′ is a bit short for 160, unless you have very conductive ground. If you have the wire, I’d try around 75′ or so; that might be about 1/4 wave on 160 for ‘normal’ soil, which is your goal. Radials in close proximity, say 60 to 120 radials, are a different story, but for one counterpoise the soil loading increases effective length quite a bit.

      As far as your radiating element is concerned, that ought to work for NVIS on 160. I prefer to avoid impedance transformers as they can be very lossy when running into a significant mismatch; I like the Johnson Matchbox tuners which are very efficient. They are old, but some things cannot be improved. You might want to see what your tuner will do with the wire run straight to it if it has a place for a single long wire.

      Another thought is to run a Zepp; this is a half wave radiating element (~250′ on 160) fed with a 1/4 wave window line or ladder line stub, which can be tapped to give a direct match to 50 ohm coax, no tuner or balun required. Simple, quick and relatively cheap. Just like the inverted L, but no radials required, and just a short coax run!

      Liked by 1 person

    2. LodeRunner

      “Currently am using a droopy dipole cut for 3910, feed point at about 39′ and ends at about 15′ above ground. This antenna works like gangbusters most of the time on 80 meters. “

      I’m not surprised. The height and configuration are very nearly optimal for 80M NVIS.

      “My tuner won’t tune my house dipole for 160m.”

      No, I’m not surprised that it won’t. And even if your tuner could match that antenna on 160M the efficiency would be really poor – probably only between 25% ~ 40%. About half of your RF power would be spent cooking your coax, and the rest would be wasted heating up the components inside the tuner itself.

      “I am thinking along the lines of an end-fed at a short elevation from the house, running 175′ of wire to a 40′ mast, with a 9:1 unun and about 50′ of counterpoise wire run out at 90 degrees to the radiator.”

      Alright, lets parse this out and do a quick estimation of “will it work?”
      175′ + 50′ = 225 feet of wire to work with.
      A dipole at 1.950Mhz is 240 feet long (468/1.95Mhz = Length in Feet)
      A dipole at 1.850Mhz is 253 feet. So, 225 is a bit short to function as a half wave radiator, but lets call it close enough (for right now).

      The ratio of lengths 175/50 is 3.5:1. So, forgetting about the right angle “bend” at the feed point for now, we have what is essentially an Off Center Fed Dipole (OCF-D), many flavors of which are also called “Windom” antennas.
      When you look at the available literature on the many OCF-D designs, the position of the feedpoint is one of the largest variables from one design to the next – the ratio varies from 3:1 all the way up to 6:1, with values very near 4:1 being the most common for multi-band configurations.

      The other variable which seems to be in most dispute WRT the “best” value, is the ratio of the transformer at the feedpoint. Some say it needs to be a 4:1 BALUN, others insist that 6:1 is the only right value, and still others use/recommend a 9:1 BALUN. Fact is, nearly all of them are wrong, including several commercial manufacturers who should know better.

      Without digging into the deeper math, I can summarize the characteristics of the feedpoint for an OCF-D with a length ratio of 4:1 — The impedance at the feedpoint of this antenna, were it in ‘free space’ would be approximately 375 ohms at the frequency where it’s reactance passes through zero – i.o.w. Z= 375+j0Ώ. When we examine this antenna over “Real Ground” at various heights, we find that:
      1. the feed impedance cycles through a set of values ranging from just under 500+j0Ώ to just under 100+j0Ώ (an envelope of approx 5.35:1), and
      2. that the frequency of resonance decreases as the height above ground is decreased, and
      3. that the median impedance decreases as the height above ground is decreased.

      Again, in summary, when this antenna is 45~65 feet above typical ground, and at it’s resonant frequency, the feed impedance will be between 200Ώ – 300Ώ. This would seem to argue for the unorthodox 6:1 BALUN which certain antenna vendors claim is “necessary” to make such an antenna work (I disagree with this claim, having never seen a 6:1 CURRENT Balun) but regardless, we still need to consider additional factors –

      Let’s now consider that 90° angle at the feedpoint. It’s well documented that as the angle between two wires (as a feedpoint) decreases, the impedance at the feedpoint is also decreased – I.E. a dipole FP in free space is 72Ώ, but if we re-orient the same two lengths of wire into a “vee” having a 90° relationship, then the feed impedance will be 36Ώ – half the angle, half the feed impedance. This relationship also applies to the OCF-D.

      Still summarizing ; with the 90° angle at the feedpoint, and the antenna still 45~65 feet above typical ground, the math says we should expect the impedance to range between 100Ώ~150Ώ – both computer models and my real-life experience have proven this to be true.

      It’s a rule of thumb that. when looking to match a source and load (feedline and antenna, respectively) greater efficiency is obtained by increasing the load impedance that reducing the source impedance. This is because lower source impedances result in greater ohmic and reactive losses. So if we want to use established and proven methods of matching our “bent OCF Dipole” to a 50Ώ line, then we should aim for a feed impedance of 200Ώ; and use a 4:1 CURRENT BALUN, a.k.a. a Guanella BALUN at the feedpoint. NOT any form of voltage balun. Ever. Period.

      To raise the impedance at the feedpoint, we can either:
      1. change the angle at which the wires meet, or
      2. we can change the length ratio between them – the shorter the “counterpoise” wire, the higher the impedance will be. At the extreme end of this is the End-Fed Half Wave (EFHW) antenna, where the counterpoise is only a foot or two long, and the impedance is on the order of 1600~2400Ώ.

      One thing we must keep in mind is that, since the wires are of unequal length, the antenna is NOT BALANCED and therefore any “voltage balun” will fail to match the load, will fail to isolate the load from the feedline (RF will be coupled onto the outside of the coax shield) and will therefore introduce serious losses into the system and likely cause other problems such as “hot mic”, distorted audio, and even improper operation of the transmitter. A CURRENT BALUN will eliminate, or at least minimize these effects.
      The coaxial feedline will ideally be 1/2WL * VF (the Velocity Factor of the particular cable) long, so that the input impedance of the BALUN is repeated at the station end of the line – this will put the least stress on the Antenna Tuner, and thus obtain the highest efficiency. AVOID Coax lengths which are an odd multiple of 1/4WL long, i.e. 1/4, 3/4, 5/4 …) because these will cause a transformation of the feedpoint impedance to some extreme value, which will put the most stress on the Antenna Tuner and maximize losses in the coax itself.

      While not a complete answer, the above should guide your thought processes to a workable solution, given you needs and constraints.

      73, LodeRunner

      Liked by 1 person

      1. Badger

        LodeRunner, good stuff & your comments/analysis always appreciated. I can confirm (till I lost a “mast” last year, aka tree branch) that the OCFD with a 4:1 CURRENT balun is a very viable solution.

        As you point out in your reply to Quietus, with the lengths not being balanced, can you address Quietus’ proposed use of end-fed with the use of a 9:1 UNUN as the connection point? Thanks!

        Liked by 1 person

      2. That’s funny you brought that up, because we had this same conversation over the weekend. Balun and Unun are acronyms- BALanced to UNbalanced or UNbalanced to UNbalanced.

        Coax cable as a transmission line is electrically unbalanced, with a dipole being balanced. So the BALUN serves as a transformer. The difference between a 1:1 or a 4:1 is the amount of feed point impedance it transforms. 50 ohm to 50 ohm, 200 ohm to 50 ohm, etc. A 1:1 comes in handy for a mono-band dipole or one cut to a specific resonance, where not much variability is expected or needed. These are also known as current chokes. A 4:1 on the other hand is very versatile, because it gives much more room to work while matching impedance values on the antenna. So for a multiband dipole, a dipole of unknown or random height, or an off-center fed (OCF aka Windom) the radio will like the 4:1 to match that 50 ohm impedance.

        Where an UNUN comes in is when we use an electrically unbalanced antenna, which an end-fed is one. A 9:1 transforms 450 ohms to 50, and is needed to keep voltage from returning to your tuner or rig. The antenna will still balance itself somehow, which is why end-fed antennas work best when grounded.

        I hope this explains it just a tad. Make sense?

        Like

  4. Ohio John

    I went the route of the 160m dipole fed with 58 feet of 450Ω ladder line up about 35 feet. I ran the ladder line right into my shack and connected to the tuner with a stub of coax. I started with a dipole that was longer than the standard 160m dipole and started trimming. I would trim a foot and go in the house and run a sweep of the bands with the antenna analyzer. I graphed the SWRs at the different band frequencies. I kept trimming the wires and measuring until I got all the bands to converge to a low SWR. My final length was about 220 feet.

    I made the center feed point and end insulators from 1/4 in acrylic. The wire is 14ga electrical wire. It is pretty omnidirectional on 160m and 80m. I am a net control station for a net on 1.900 and get checkins from Oklahoma to Maine and from Canada down to Florida. I’m here in Ohio. Most of the guys on the net are running inverted L and listening on a beverage antenna.

    I went with ladder line because of the much lower loses than coax. This is a link to a site with a pretty neat line loss calculator. It factors in SWR, feed line length and feed line type. It then tells you what your final power out will be for your working conditions.

    http://www.qsl.net/co8tw/Coax_Calculator.htm

    The same site also has some antenna analysis that shows what your lobes and launch angle look like for dipoles that range from 0.05 up to 4 wavelengths above ground. Good information.

    http://www.qsl.net/aa3rl/ant2.html

    73,
    John

    Liked by 2 people

  5. Good article, Loderunner, you obviously put a lot of time into it. Your follow-up and JohnnyMac’s article made me think about this some more. I’ve recommended inverted L antennas to several friends as a solution for working DX on the low bands from small lots, so I was particularly impressed with the detailed information on how to tune the Inverted L on multiple bands with only a variable cap. That was excellent information; not needing a fancy tuner is a big plus anytime, and especially in austere circumstances. Good stuff!

    There are a few things I’d point out for folks to keep in mind with specific regard to NVIS antennas in a grid-down situation:

    1)The low angle of radiation from the inverted L on 160 is not likely a big problem during the daytime, when the D layer attenuates the low-angle RF, but it has the potential to be a problem at night when the D layer is gone. That low angle is susceptible to DF, where a Vee or a dipole has less low-angle RF. See the NVIS part 3 article posted elsewhere on this site, link- https://brushbeater.wordpress.com/2016/08/15/nvis-explained-iii/ Under ‘normal’ circumstances, that low angle radiation is a good thing, resulting in DX contacts on 160, but in a grid-down situation, it could be a problem. I would not use this antenna for NVIS on 80 or 160 at night in a grid-down environment. HF DF is not hard to do….

    2) wire required for a good inverted L is greater than the requirement for a dipole- with 200′ for the radiating element, plus anywhere from 300′ to 750′ or more for radials, that’s around 500 to 1000 feet of wire. A dipole for 160 is around 250′, give or take, so the inverted L takes 2 to 4 times the wire. 500′ spools of 14 ga THHN runs around $40, and copperweld is in the same neighborhood, so it’s not a lot more money now, but that’s another thing to keep in mind. If you think you might want to build this, get the wire now while you can.

    3) as a fixed station antenna, especially on a size limited lot or where stealth is needed, the inverted L has potential, but it doesn’t look as though it would be as quick to put up as a low altitude Vee or Inverted Vee. I’ve worked with temporary radials for Field Day low band vertical antennas. Once. Radials can be a headache to deploy and re-pack, and take time under the best of circumstances. For portable operations, the inverted vee is hard to beat for speed and ease of deployment, and the Vee is almost as fast. With a linked dipole setup, the weight of the antenna with 18 gage stranded copperweld is bearable even for 160.

    I told Johnny Mac I was going to put up an inverted L and report back; I’ve got the room to do some A/B/C comparisons with Vee and inverted Vee antennas for NVIS.
    I’m interested in what I will learn.

    warm regards, Loderunner!
    Keypounder

    Liked by 1 person

    1. LodeRunner

      Keypounder,
      With a 1/4 WL Inverted-L, you definitely need a lot of ground radials. This is because the feed impedance of such an antenna is so low. But as the length of the antenna element increases from 1/4 WL towards 1/2 WL, the feed impedance rises rapidly. With ‘ground losses’ remaining the same; as the feed impedance rises, so does the efficiency of the antenna. This is a real plus, because it means that you don’t have to lay out dozens of 100+ foot radials to have a highly efficient antenna system: 3 radials is minimal, and 8 is ideal, just as I diagrammed in my article.

      As for susceptibility to DFing, on 160 meters, the ground wave is negligible – at least 10dB below the NVIS signal level – so if you’re using minimum power necessary to achieve communications then the ground-wave should be scattered/absorbed within 10 miles or less. In point of fact, an inverted vee or low dipole has just as much groundwave as the Inverted-L and perhaps more, depending on soil conditions and re-radiation from other elements of the environment, I.E. utility poles, whose ground leads act like top-fed vertical antennas.

      80 Meters on the Inverted-L has a bit more ground wave (because one of the current loops is in the vertical section of the wire, just above the feedpoint), but that’s one of the things we take into account when we plan our communications windows, right?

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