Hello OM's, hi Gil,

I'm trying to make an effective endfed antenna
for 20m band.
I don't know why everyone assume that 1/2WL endfed
wire has a 2450 ohm resistance?

I tried in different places with about 10m
long wire (1/2 wavelength at 14Mhz)
mounted vertically on a fishing rod from
fiberglass like this:



Each time the measurement (with my trusty NanoVNA-F, OSL Calibrated)
of the parallel resistance Rp (orange line)
at resonance (Xp = 0, blue line) is about 2100 ohms - as here:



note that at 7.2Mhz (1/4 WL at resonance Xp = 0)
the wire has a resistance Rp of about 120 ohms.

This leads me to think that the transformer for my antenna
for 20m should be 1:42 (not 1:49 or 1:64) !

You could just take 1 secondary turn off a 49:1 BALUN to get 42:1. I have tried both 49:1 and 64:1 BALUNS with my end-fed antennas and have not noticed much difference between the two. So I don't think there would be much practical difference between 49:1 or 42:1.

PS: The impedance of an antenna is dependent on its proximity to the ground and other objects. The theoretical impedance is for a horizontal dipole, 1/2 wave length above the ground. Since you measured with the antenna oriented vertically, it makes sense the impedance would be slightly different.

of course you're right - making waves is easy and if we work with 100 watts
there wouldn't be much of a practical difference, but my goal is a maximum effective ENDFED antenna -
working in a field with a 5 watt QRP transceiver  - any decibel loss make a difference.

It is logical to discuss the transformer itself -  the most correct option
for winding 2,3 or more primary windings, which is the low loss material for 14Mhz?

I decided to try a few different materials and a different number
primary windings.



In a 50 ohm circuit we can estimate the core efficiency
measuring parallel resistance Rp of the primary winding
such as 1-50/Rp, and the results for 14.16Mhz are as follows:

4 turns without core (air) = 352 Ohm -> efficiency 86% (~ 1db loss)
3 turns, material 33ri = 341 Ohm -> efficiency 85%
2 turns, material 43 = 210 Ohms -> efficiency 76% (~ 1.5dbloss)
3 turns, material 43 = 452 Ohms -> efficiency 89%
3 turns, material 52 = 526 Ohms -> efficiency 90%
4 turns, material 61 = 2862 Ohms -> efficiency 98%
3 turns, material 4c65 = 2980 Ohm -> efficiency 98%
3 turns, material 77 = 93 Ohms -> efficiency 46% (~ 3db loss)

Logically, the choice here is Ferroxcube 4c65 or 61 material.

I decided also to wind a normal transformer with separate primary and secondary windings
with equal length of wires to form a 1:1 transformation - ie. 50 ohms
to be transformed into 50 ohms :



You can do the same simple exercise at home using a 50ohm dummy load to check VSWR at different frequencies.


At the end my transformer looks like this :



with 3 turns primary and 19 turns secondary on 4c65 core.

Hello. True, there isn't much difference between 49:1 and 64:1. It is a high dividing number, so there will be only a few Ohms difference in the end. Truth is, the impedance of a half-wave wire is very difficult to measure and theoretically should be infinite, but it isn't... VNA measurements are just not reflecting the truth... The resulting impedance and divider will tell you approximately what it might be. Ultimately it doesn't matter. Make your wire as close as possible to a half-wave (don't forget the velocity factor of the wire) while maintaining an SWR lower than 2:1.
Try to wind your transformer the way I do it, using a single wire and twisted primary turns. Three turns primary for low bands, two for upper bands above 20m. 43 and 52 material cores should work best, 43 for lower bands, 52 for upper.

Gil.

Quote from: gil on May 27, 2021, 06:16:21 AM....Truth is, the impedance of a half-wave wire is very difficult to measure and theoretically should be infinite, but it isn't...

I was thinking about this statement. An end-fed antenna is an unbalanced antenna, because one end of the antenna connected to the shield of the coaxial cable through the UNUN, and the shield is grounded. The impedance cannot be infinite because there is an electrical path to ground.

I honestly don't believe the EFHW is the most efficient antenna for QRP operations, and trying to get 5% more out of it is kind of a science project without much real-world benefit. Personally, I like using a doublet with balanced feed line to a multi-Z tuner for QRP work. Maybe it's more efficient, or maybe it's the placebo effect, but I feel it works better most of the time. I posted on that antenna here earlier. After watching your videos, I think the next step to improve on the doublet would be the Delta Loop fed with balanced twin-lead line, especially on 10 m, where the size is small enough to be practical and the band is now starting to open up regularly.

Experimenting with antennas is part of the fun of QRP operations. Build lots of antennas, try things out, and see what works and doesn't. That's my favorite part of your YouTube channel. You aren't afraid to experiment and have given us lots of ideas.

Hello again,


I did forgot to introduce myself - I'm Asen, LZ2DM.
I live in Varna - and I take this opportunity to invite you
to visit my city :)

Friends - I'm glad we have a discussion here
to meet theory and practice together.

Measuring high impedance's is very difficult
and someone can write a dissertation
on this, but I believe and trust my NanoVNA
(in the range of 3 to 15Mhz this is relatively easy) :




I decided to model my antenna in Mmana-Gal software anyway:




The result for the wire impedance (as expected) is  ~ 2100 ohms -

I wound two transformers with the same core size
- one with 4c65 material wound in my way with 3 primary windings and ~ 19 secondary
windings - just like an ordinary transformer. My idea is to use
the small white cable that will provides DC ground (anti-static) and
to ensure the return current in the  wire antenna circuit through this cable.
I guess that this way there will be less current return from the antenna
in the RG58 cable.

The second transformer is wound in the standard way with 43 material
like 1:49 with a 3kV capacitor, as popular and everyone suggest :



First let's check the VSWR of both two transformers:




Both are perfect - 4c65 is spot on with VSWR 1.0, and 43 is VSWR 1.1.

Does that mean they are effective?

I decided to check what are the currents in the wire, using
DIY current probes that will give me relatively right results
for comparison :




Transformer losses wound on 43 material
according to the most popular scheme are nearly 50%!
Looks like my calculations above are correct.

And where does this energy go?
Let's see with the thermal camera:

in the beginning - 43 material:


In the beginning - 4c65 material:


After 10 sec. and ~ 50W of power applied capacitor and transformer
with 43 material become hot and cannot be touched by hand:




While the one with 4c65material  (and probably 61 material too)  even after
very long operation does not heat up at all:



what do you think OM's ?

In the next few days i will try to use my tiny Spectrum analyzer and check all again - this time with decibel comparison.


73

It looks like you are driving your -43 UNUN into saturation at 50W. According to the chart on Page 25 of: End Fed Half Waves - Steve Dick K1RF, a single -43 UNUN can take 15W continuous power. So I'm thinking your antenna is emitting about 15 W of RF energy, and 35 W is being transformed to heat. If you repeat the test with less than 15 W, your UNUN should be closer to 90% efficient. I am building an UNUN with two stacked FT240-43 cores that can, according to the linked PDF, take 85W continuous power. CW and SSB are not continuous duty cycles, so the max power rating would be 212W and 340W respectively.

Keep posting and let us know your results. I don't have the ability to conduct any measurements, so have to build my antennas based on references and expect they perform up to spec.

73 de Brian KM6ZX

Thank you so much for taking the time to post your results, very interesting! Of course the EFHW is not the most efficient, but efficiency is not always the primary objective. I like the practical nature of the antenna. Interestingly I just tested a 49:1 with an FT-240-61 core, two turns primary, and it did not work on Hf or 6m. Maybe I did something wrong, bad solder joint or something, but the SWR stays around 5:1.... The same one using an FT-240-43 works great... What do you think of that?
Gil.

Quote from: gil on June 01, 2021, 02:56:49 AM....Interestingly I just tested a 49:1 with an FT-240-61 core, two turns primary, and it did not work on Hf or 6m........ The same one using an FT-240-43 works great... What do you think of that?
Gil.

Gil -

The document I linked above, The End Fed Half Wave Antenna by Steve Dick, K1RF - page 26, common problems are, "Using wrong toroid materials. Stay away from type 61 ferrite cores and any powdered iron cores like -2 or -6. Primary inductance too low and it won't work." Which explains your result.

Page 24 of the same document, "More than 1 toroid generally improves efficiency: Single FT240-43, 2T/14T wind is 66.5% efficient on 80M and has inadequate primary inductance/high VSWR. For a 100W rig on CW, dissipates 14.7 watts – much too high." Which matches Asen, LZ2DMs, observation above.

Another resource I use is http://toroids.info.

73 de Brian KM6ZX

Hello friends,

today I decided to continue tests with the two transformers
4c65 and 43 - but only with 5W QRP, because at higher power
the transformer with 43 material has losses as seen above.

The first test is in CW mode, and the results from RBN network
are practically the same:




During the test at a ~30m distance from the antenna and using
my tiny Spectrum Analyzer the measurements showed again equal
result for both transformers at 5W -37dBm:



I did the second test with SotaBeams' WSPRLITE with only 100mW:



I achieved very good distances with both transformers
but still approximately the same results:



From the tests so far I made the following conclusions for my self:

1. It is good to know what is the resistance of the endfed wire
so that we can fed it as well as possible.
2. Material 43 is usable - but it is good to have at least
three (or more) primary turns to reduce its losses.
3. The 100pF capacitor gives additional losses.

However, all this is of no practical importance with 5W QRP.


73

Hi, so, do you conclude that 4c65 is better than 43?

Gil.

Hi Gil,

I am far from being an expert and can't give opinions
what is good and what is not - I came here because
i like to be outdoors with my radio - just like you.

I'm watching your videos with interest - that's where
it all started for me.
Admiration for what you do - for popularizing our hobby.

My usual position is on the beach -
I can't stretch wires back and forth there,
because there are a lot of people walking around,
but with a raised fishing rod (which still has
wire on it) - I do not look suspicious.
 
In my attempt to make more and more dx connections
I found that increasing the power the connections
did not get better.

So I got to ask here, and to show what are my observations
until now.

I'm counting on our community and you Gil.


73 de LZ2DM

QuoteI found that increasing the power the connections did not get better.

That my friend is a very important statement...
Thank nyou for sharing. Keep experimenting!
I will too :-)

Gil.

Quote from: gil on June 08, 2021, 03:55:20 PMThat my friend is a very important statement...

I recently upgraded my K2 with the KPA100 internal amplifier. I built a new UNUN using two stacked FT240-43 cores. According to the reference from K1RF it can handle 300W PEP. I tuned into the Utah Web SDR (ca. 500 miles from my QTH) on 20m and keyed down for several seconds. I saw a very strong S9 + 60 dB signal, loud and clear. It was by far the strongest signal on the display. To make a long story short, I incrementally reduced the power down to 0.1 W. Even at 0.1 W, I still saw a nice clear S5 to S6 signal on the SDR. I could have worked any station in Utah that night with 0.1 W as long as the receiving station's noise floor was low enough.

Assuming that the band is open, there are only three reasons I found QRP doesn't quite get through. There may be more, but these are the ones I noticed:

1. The far field radiation pattern is not the ideal pattern and the station lies within a null. Unless your dipole or EFHW is horizontal and 1/2 wave length above the ground and free of obstructions, you have nulls.

2. Something is defective in your equipment and the antenna isn't radiating at all.

3. The receiving station has a very high noise floor, and your signal falls below it.

When does QRO power help? It helps only in two cases that I can discern:

1. Getting your signal above the noise floor, especially for stations fighting an S8 or S9 noise floor. This seems more common on SSB than CW.

2. Marginal band conditions. Having more power, your signal may only drop from an S9 to an S7, which is better than a QRP signal dropping from an S5 to an S3 and below the receiving stations noise floor.

Should we ditch our amplifiers and go QRP only? Outside of a prepping situation, probably not. The other thing I learned from my little SDR experiment is with each power reduction, I did see a signal strength reduction on the SDR. That tells me two stacked FT240-43 toroids can handle 100W CW without saturating. Otherwise, there would have been no change in the SDR reading until I got below the saturation threshold.

My EFHW can handle 300W PEP, but it's still a compromise antenna. It is too low to the ground and too close to buildings to have a predictable far field radiation pattern. I have no idea what the radiation pattern really is. I need to either get it up 10m in the sky, which isn't really feasible at my QTH, or construct an antenna that doesn't need to be mounted as high. I am considering a delta loop in this case. Further down the road, I would like to build a portable 2 el Yagi and mount it on a 10m military fiberglass push up mast. Probably not the ideal antenna for prepping, but would be a great antenna for field day, camping, or natural disasters and other emergency comms. My intent is to try to get away from using a compromise antenna so that the performance will be more predictable.

73, de Brian

Here's a photo of my 49:1 UNUN consisting of 2 FT240-43 stacked cores. I built it according to Steve Dick, K1RF's reference on EFHW antennas.

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