End-fed Halfwave Matching Transformer

[HomerBB]

Good for you!
Gonna go at it with a vengeance.
I've tried a few before, but only got "close".
The test response from this one has been the best, yet. As weather fairs and opportunity presents I'll get it done.
BTW, what is your most used antenna setup for SOTA?

 

[binrat]

Homer,
For SOTA, it is about 60/40 EFHW / dipole.
I am fortunate to have many of the summits with a trees or at least a few bushes. EFHW work well in an inverted V configurations, ends held up with hiking poles and the center via a crappy fishing pole.
I plan to finish today another EFHW that will weigh a few ounces for 20 metres and is mounted on a male bnc connector.
If I get bored today I will post some pictures of various transformers.

 

[binrat]

Well my latest 20 metre efhw is done. Toroid is a Fairrite # 2643625002 wrapped with #24 wire 49:1 ratio, mounted on a BNC. Wire element is #26 poly stealth wire.
SWR was 1.09:1 at 14.160.

 

[HomerBB]

View attachment 43221 Well my latest 20 metre efhw is done. Toroid is a Fairrite # 2643625002 wrapped with #24 wire 49:1 ratio, mounted on a BNC. Wire element is #26 poly stealth wire.
SWR was 1.09:1 at 14.160.

Awesome.

 

[magnuman]

I wanted to make a QRP 49:1 autotransformer for my travel pack.
The previous builds made with FT240-43 were larger than I wanted or needed. Besides that, I was bored seeing the weather had shut me in. This time I used FT140-43. Instead of a 2/14 turns method, I went with 3/21 turns, both being 1:7 ratio, or 49:1.
With 3000 Ohms resistor (3×1k in series) it tested out really good in all the bands I want to use in the field. I will be tuning a wire soon.
I needed a box, and was out of those I prefer, so I fell back only my repurposing tendencies. I had 3 boxes of this style, and use only one.
2WR183_Homer
K5HBB

View attachment 43210 View attachment 43211 View attachment 43212

Hey Homer,
Just wondering..in your picture you have the earth of the coax connector connected to turn 2 of the coil and the center to turn 0. I have always put the center of the coax to turn 2 or 3 of the coil and the ground at 0.
Probably makes no difference but thought I would mention it.

 

[HomerBB]

Hey Homer,
Just wondering..in your picture you have the earth of the coax connector connected to turn 2 of the coil and the center to turn 0. I have always put the center of the coax to turn 2 or 3 of the coil and the ground at 0.
Probably makes no difference but thought I would mention it.

You, my friend, are only the second person to notice this, and bring my attention to it.
I have corrected it taking reversing my mistake. I have made them correctly, but ... my fallibility ...
As good as the SWR was before, it is now slightly better on the work bench. I have yet to test it with a wire. Soon.

 

[magnuman]

You are welcome Homer...
I think you like experimenting so when you wind the secondary leave about 6 inches of wire on the end after the coil. In the field especially for SOTA stuff we don't always get to set up the antenna the way we would like. Having the ability to add or remove a winding on the secondary can bring the antenna into perfect tune for max performance on any one band. The height above ground of the transformer and its proximity to anything conductive also affects the SWR. I don't run any grounds or counterpoise from the unun but I do place a choke in the coax line, 0.05 wavelengths for the lowest band in use down from the unun.
These end fed half wave antennas using the 49:1 unun transformers are the easiest and most practical antennas out there. And they work well. Single band or multiband. You should not need a tuner at all.
My vertical 40m version on a 60 ft push up fibreglass pole works awesome.
No ground plane to worry about.

 

[HomerBB]

I am looking forward to getting one to work "no tuner". To date, each I've tried has been "close", but never tuner free.
I've been running a random endfed on the base as I was using a tuner anyway.
Given your advice about winding, or unwinding a wrap of wire from the toroid for fine tuning a band, what prohibits adding a wrap or two, then putting tap points across the last five wraps for a quick change setup?
Your thoughts...? Anyone...?
I am going to do it and see.

 

[binrat]

You, my friend, are only the second person to notice this, and bring my attention to it.
I have corrected it taking reversing my mistake. I have made them correctly, but ... my fallibility ...
As good as the SWR was before, it is now slightly better on the work bench. I have yet to test it with a wire. Soon.

I noticed but thought it was by design. My small ones you need to do that due to the limited working space in there.
I have never had a good wide scan with a resistor inserted in leu of wire. I add wire for 40 metres, tune it to the best swr by adding or removing turns off of the toroid, then tune the wire to the part of the band I want. Then check other bands (20,15,10) and add the compensating coil about 72"-74" from the transformer and tune the coil. It seems to work for me.

 

[HomerBB]

@binrat
The resistor thing is just to "ballpark" the transformer for me. All of them I've made that passed the resistor test failed the wire test. This time I will take more time and use the methods you guys are providing to get where I need to be.

 

[magnuman]

I am looking forward to getting one to work "no tuner". To date, each I've tried has been "close", but never tuner free.
I've been running a random endfed on the base as I was using a tuner anyway.
Given your advice about winding, or unwinding a wrap of wire from the toroid for fine tuning a band, what prohibits adding a wrap or two, then putting tap points across the last five wraps for a quick change setup?
Your thoughts...? Anyone...?
I am going to do it and see.

I did see something about that on the net and also using a variable capacitor across the input. I tend to add or remove turns when tuning for lowest SWR in a single band EFHW.
I have used the following criteria when using end fed antennas.
Please take into account that anything conductive nearby the antenna will serve to detune or tune the antenna to a certain degree. They really need to be in the clear. Raising and lowering the transformer above ground will affect your SWR readings. For best results try to get the transformer in the clear and 4m or so above ground.
For 80m to 10m use a 49:1 ratio and you will be in the ballpark. For 160m and 80m only, you can try a 64:1 ratio which has 3 turns on the primary and 24 on the secondary.
The outer shield of the coax is part of the antenna system (whether we want to admit it or not.) For half wave end fed antennas it is said that 0.05 wavelengths of counterpoise is enough. So I always place a choke 0.05 wavelengths in the coax down from the transformer for the lowest band in use. That works wonders to keep RF out of the shack and really makes tuning a lot easier. I have not tried grounding the transformer. People have said it helps performance.
Tune it to 14.2mhz by adjusting the wire length.
To bring in 10 and 15m add the 6 turns on 1" a diameter former in the antenna wire, about 2m from the Transformer.
How you lay out the antenna also determines it's effectiveness. Sloping or vertical or inverted V works well.
Its a half wave multiband antenna. CBers and Hams have been using them with great effect on 11 and 10m for years. they work great on the other bands too!