Short EFHW design: loading coil and pigtail length

Equipment Antennas
21

The inserted coil has two different functions.
At 7Mhz it acts as an extension coil, the well-known principle.
At 14Mhz it acts as a resonant circuit, a trap.
What is special about this trap is that the capacitance is provided by the wire ends and coil winding. There is a detailed calculation formula from OE5CWL/OE6CWL for the inductance that achieves the correct shortening factor for 7Mhz and the resonant frequency (14Mhz). The wire length to the right of the trap plays an important role here.

The inserted coil has two different functions.
At 7Mhz it acts as an extension coil, the well-known principle.
At 14Mhz it acts as a resonant circuit, a trap. What is special about this trap? The capacitance is provided by the wire ends and coil winding.
There is a detailed calculation formula from OE5CWL/OE6CWL for the inductance that achieves the correct shortening factor for 7Mhz and the resonant frequency (14Mhz). The wire length to the right of the trap plays an important role here.
EDIT
OE6CWL called these traps CWL traps and used them with dipoles. PA3HHO now had the idea of ​​using these CWL traps for end-fed antennas and presented the 1:64 transformer.
EDIT

i have an excel file to calculate. With this I was able to successfully calculate antennas for 20/15m and 20/10m. 60/40/20/10m or 30/20/10m would also be conceivable, with the extension coil working for 60m or 30m
73 Chris

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22

That’s great! Would you be able to upload the excel file somewhere? Also, would you mind sharing what changes are needed for the configuration in your figure if one wants to use a ~5m wire instead of 10m? Thanks!

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23

I say again, I never use 9:1 transformer. I post this only as something to think about. Is it good or not, I do not know.

6,5m of wire is not resonant on any band. Resonant is the wrong word.

Instead, it shows around 450 ohms impedance on all bands. With 9:1 transformer it shows 50 ohms to tx.

Efiency will be bad. Most power is transformed into em waves with resonant lenght of wire

24

The principle allows only the following

The piece of wire to the left of the trap defines the second lowest band. In the example, the band is 20m and the length is half a wavelength, i.e. 10m.

This wire can also be operated on multiples of the frequency 14Mhz, i.e. 28Mhz 10m here.

In addition, I can use the extension coil/trap to bring a lower band into resonance. In the example 40m with 7Mhz.

73 Chris

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25

To clarify what DL1CR said: Only the 10m wire gives you two bands (20,10) + coil gives you a single 3rd band (40) without jumpers. The wire to the left of the coil must be ~N half waves.
So all shorter wires need a specific loading coil value for each band. A tapped inductor and crocodile clip would do this.

I am looking at a vertical EFHW, so I also want ~5 m long. For the 20m band it models thus:
feedxfmr ----4.4m---- 23uH — 0.8m ----

OK so did some more modelling of a 2 band 20/10 short EFHW

feed ---- 5.6m — 19uH — 1m —

This can be made to work on (17 or 15 or 12) /10 by shortening the 1m wire to tune.

Loading coils are only approximately inductors, so don’t expect it to “just work”. It is much easier to have an antenna analyser or vna to adjust multiband antennas.

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26

Check out N0LX’s webpage, he has several designs at https://n0lx.com/
They might be just what you are looking for.

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27

Thanks for all the helpful replies! I have already built a 10-20m EFHW using a wire approx 10m long. Now I think I can shorten this antenna to 6-7m wire with a loading coil.

Could someone share some insight about how to come up with coil specs and wire lengths for arbitrary HF bands? Surely, there is some experimentation but it’d be great to learn the theory behind it.

I also wonder if this experiment can be extended to even shorter wire lengths? As I’m interested in highly portable use-cases, I am asking myself: what is the shortest wire length one can use with max 2-3 loading coils to work HF bands? Can a wire shorter than 3 meters be used to work the 10m band? How would it look, and what loading coils would be needed?

Thanks for your suggestions!

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28

You’ll find the formulae and limited explanation for inductively loading dipoles in section 6-6 of the 1994 ARRL Antenna Book.

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This may get you started. No idea if it’s “right”. Loaded Quarter Wave Antenna Inductance Calculator – M0UKD – Amateur Radio Blog

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30

G3CWI
That works great for a 1/4 wave but the OP is asking about a Half Wave so the calculator doesn’t work. I played with it trying to see if it could be ‘hacked’ to work but no luck.

HOW TO has been provided by DK3IT, DL1CR and myself KE6MAK. The answer is a 35 uH choke/loading coil at the end of his 20M wire.

73,
Howard KE6MAK

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31

Paul,
About the shortest you will get is ~ 40 feet for a shortened 20/40 that also works on 10M.

If I may suggest; ARRL Antenna Handbook may help. Though the info on loading coils is based on a 1/4 not a 1/2 wave antenna. The 35uH coil used that added to a 20M EFHW and ~ 6-7 ft of wire also serves as a choke for below 40M to block lower frequencies. You would need a similar coil/choke to shorten your 20M section and to be honest…I have not found the answer for that. I will say that 40 feet for 40 meters is about as short as you can get and still have reasonable antenna efficiency.

I understand limited space, one option is to run the antenna as a vertical. I have done that with the Spiderpole 12M pole and my shortened (10)/20/40 which is about 39 ft. And if resonant on 20 and 40 it should tune on 10 as well.

If you would like more assistance, my contact info is on QRZ and I am willing to provide what help I can. Am also in SoCal.

73,
Howard KE6MAK

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32

Sugestion

You do not need to put your wire in straight line. It can be Z, W, L, whatever suits you. Verticaly, horizontaly, mixed. It will affect impedance, nobady knows how

33

I am trying to design a short end fed resonant antenna. I see on N0LX.com a few designs, and comparing them to the values for a coil loaded dipole at Coil Inductance Calculator the values seem pretty comparable. This makes me think that I could just use that calculator to design an antenna that would fit my space and instead of feeding it in the center, I could make it a continuous wire, and feed it at the end. Is this thinking correct?
This also got me thinking, it would be nice to have just one coil for the antenna. I could not find any calculators for this, but think that if I design a dipole for a frequency of half of what I want, (ex. 7.00 MHz if I want a final frequency use of 14.00 MHz) and then only build one side of the dipole, and feed it at the end, it should work, or just the calculations for a ¼ wave vertical. (Half the freq. = double the length) From a popular design for a coil shortened 40/20/10 meter efhw, I see that at some point the inductance would behave more as a choke than as a loading coil. What formula would I use to avoid the inductor becoming a choke?
To take it to extreme, could I feed a 40 meter mobile whip through a 49:1 unun and use it as on 20 meters without radials? I know the impedence would be different, and maybe a different ratio unun would be needed, but food for thought…
Thanks for any direction from those with more antenna wisdom than I

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34

btw: My (rather anecdotal) experience with this design is that it performs rather poorly on 40m.

The only advantage over a loaded up-and-outer is that the feedpoint impedance is much less variable wrt to ground and type of deployment.

35

Go on the air as fast as possible. Put 9,9 metres of wire on the 49:1 transformer. That is end fed. There must be a condensator of 100pF on primary. That configuration will give you 20 and 10 meters.

After you put that use it on. 20 and expecialy 10m are day bands. Do qso s during the day and calculating at night

I tried 9,9 m of wire with coil. On 40 m it gives me about 20 kHz of useful range without tuner

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36

I don’t like giving advice on exactly how to do it. But please explain the background.

The shortened EFHW for 40, 20 and 10m can be imagined as follows.

A theoretically 20m long dipole that is fed in at the end of one half. At this 10m long half the dipole is unchanged.

The other half is greatly shortened with the charging coil, similar to a mobile antenna. The coil is calculated accordingly. Severely shortened antennas always have a very narrow bandwidth and must be carefully tuned. In addition, the charging coil must also be tuned as a trap for 14Mhz.

If you stick to the length and inductance specifications, fine-tune for the preferred range in the 40m band, and hang the antenna freely, the concept is brilliant.

73 Chris

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37

Hi all,

I was reading this thread with interest.

The initial goal of @KK6FJD was to build a resonant EFHW using his 49:1 coupler. It should be max. 10m long and support at least the 20m and 10m band. He further asked for the theory/calculation of a possible loading coil to alternatively support a lower band, e.g. 40m.

There were many answers, but only a few actually complied with his requirements, especially the most critical one, which is the max. length of 10 meters (I don’t know its configuration though).

Let’s take a step back. A 10m long radiator wire would fulfill 100% of his hard requirements: it’s max. 10m long and together with the 1:49 coupler it should be resonant on the 20m and 10m band. It’s that simple and no coil is needed.

It kind of reminded me of the old illustration What the customer wanted.

Actually, I don’t have an answer for the calculation of an EFHW loading coil, since all the online calculators I know of are designed for quarter wave verticals or center fed dipoles.

When I built my 7-band EFHW with one bypassable coil, I had exactly the same problem (page 37ff explains a bit about the tuning procedure). I used these calculators to guess the inductivity and realized soon that the needed inductivity was well below the calculated one, i.e. 25uH instead of the calculated 35uH for the 60m band. So at the end, my solution was to experiment and observe in the field.

I was hoping that somebody could come up with a better solution, and I think @ZL1THH showed a good alternative by modeling the whole antenna, including its coil!

I have a kind of love-hate relationship with 4nec2 and definitely should try another modeling software when time permits. In the long run, the time is well invested.

So at the end, if there is no ready to use EFHW coil calculator available, either experiment in the field or try a decent modeling software.

73 Stephan

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38

Hallo Stephan,
ich habe dir eine Nachricht über den Reflektor geschickt.
73 Chris

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39

Thank you all for your responses, I guess I will just experiment, and see what works…

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