An End-Fed Wire Antenna for 80m.

[KK0G]

So what was the end fed tuner kit you ordered?

[gil]

So what was the end fed tuner kit you ordered?

Hello,

The kit was a BetterQRP end-fed half-wave tuner for 40 to 15m. That is the one I took camping. It does not cover 80m. The 80m end-fed matchbox I built was from scratch... I need to stretch the wire to measure the SWR, because with the wire over the house (inverted U), I get too high a SWR...

Gil.

[KK0G]

So what was the end fed tuner kit you ordered?

Hello,

The kit was a BetterQRP end-fed half-wave tuner for 40 to 15m. That is the one I took camping. It does not cover 80m. The 80m end-fed matchbox I built was from scratch... I need to stretch the wire to measure the SWR, because with the wire over the house (inverted U), I get too high a SWR...

Gil.

Nice looking unit, pretty similar to my SOTA tuner. In my opinion the EFHW is the key to lightweight, quick and easily deployed field antennas. Dipoles, loops, etc are great permanently installed at home but they suck to put up quickly under field conditions, not to mention the hassle of carrying feed line.

[gil]

Nice looking unit, pretty similar to my SOTA tuner. In my opinion the EFHW is the key to lightweight, quick and easily deployed field antennas.

Exactly. I do have a SOTA tuner as well, but I find the end-fed to be much easier to set-up.

Gil.

[gil]

Last night I tried the match box with a 4.7K resistor as a dummy load. 6:1 SWR! I don't understand... I should be getting a low SWR with that value. Everything is done exactly as VK5ZVS' site describes. The only difference it seems is that I use an aluminum box instead of plastic. I even tried removing and adding turns on the primary, changing the feed point, and it always made it worse. Where did I go wrong?

I haven't been able to string out the 135' of wire yet for testing, except once with the wire going over my house, a really weird dome shape, and I got a hight SWR too, though more around 3:1.

Gil.

[Archangel320420]

A shot in the dark here, Gil. I am not certain that the SWR would be high using a dummy load in that the resistor is certainly not resonant. The reading is correct. The RF is going into the resistor but does not like it since it is not resonant.

Also, what ohm is your feed line for the End-fed Zepp?

[Archangel320420]

I  meant... The SWR should be high in this situation.

[gil]

Also, what ohm is your feed line for the End-fed Zepp?

Not a Zepp, per say. Fed by 50-Ohm coax.

Reading this right now: http://www.aa5tb.com/efha_wrk.html
The site had great articles: http://www.aa5tb.com

Gil.

[Archangel320420]

WHOOOOAAAA   Everything you ever wanted know and not know about end-fed antennas  :)


http://www.eham.net/ehamforum/smf/index.php?topic=65832.0

[Archangel320420]

WOW.  After reading some of those links, Gil, I am as confused as ever. haaaaa.

I used to run a 65 foot length of wire, end-fed, with 300 ohm ladder line. I used a match box between the transmitter and the ladder line to match the transmitter. I would say the ladder line was 20 feet long or so up to the antenna. It performed very well especially on 40 meters, but I think the SWR was 1 to 1 only because of my matchbox, not my antenna. I believe end-fed antenni are around 400 ohms at the feed point. The other thing about counterpoise is my station was located on the 2nd floor of my home and I had a long wire running to ground. All my equipment was attached to that ground and pehaps that is why my particular antenna worked so well. I see better what you are trying to do now from the links you posted, Gil. Thanks. Interesting stuff.

[gil]

Well, I read impedance is closer to 5K. I made a 9:1 toroidal transformer tonight, 3-turn primary, which I connected to a BNC, and a 27-turn secondary in parallel with my 100pF fixed capacitor and a 4.7K resistor. Using my "Tenna Dipper," I got a dip at 5.15MHz! Bad for 80m, but perfect for 60m! Which makes me suspect the information posted on the site I used for my (presumably) 80m matchbox, which uses a total of 34 turns with a tap at 7. It's a 3.8:1 transformer, right? So, how could that work?

I did try to remove turns on the secondary and got:

Pri	Sec	Ratio	Freq
30	3	10:1	5.15
29	3	9.6:1	5.15
28	3	9.3:1	5.3
27	3	9:1	5.45
27	2	13.5:1	5.32

The 13.5:1 result is surprising. The Tenna-Dipper isn't a precision instrument of course.. It does show however that either 100pF is not going to work for 80m, or my 4.7K resistor is the wrong value to use. With a variable capacitor, getting a dip on 3.6MHz was no problem, but I don't have a capacitance meter, so I'm SOL there.. I emailed VK5ZVS (http://www.spirat.com.au/vk5zvs/pic54.htm) for some clarifications. On the photo she has posted there, the toroid looks like it has closer to 60 turns as opposed to the 34 mentioned in the text... I did get a dip around 4.6MHz, and the LED is not entirely lit below 6MHz.. Not sure what that means.. I'm going to have to find a way to stretch that darn 135' wire!

I am getting confused too  :o

Gil.

[cockpitbob]

Hi Gil,

Here's how I look at EFHW tuners.  Two things have to be right:
1)The capacitor and transformer's inductance have to resonate at your desired frequency.  Nothing will work right if that L-C circuit isn't tuned for your operating frequency.
2) The turns ratio of the transformer must be right to match the impedance of the resonant 1/2 wave wire to 50ohms.
Actually, there's a 3rd:  the antenna wire also has to be resonant at the desired frequency.


Here's where it gets tricky for you.  If you add or subtract turns you change both the turns ratio and the inductance, so the resonant frequency changes as well as the turns ratio. 

I've never done an 80M EFHW but I've designed a lot of 10M to 40M ones and 4.7K 2.5K is about right for their antenna impedance.  VK5ZVS's design has a turns ratio of only 4.8:1 which will only transform 50ohms to 1.18Kohms ??? .  Maybe an 80M wire is so close to the ground that its impedance is lower than a shorter wavelength antenna, but 1.18K suprises me.

Here's what I would do:
- With a 1.2K resistor and the variable cap, tune your box to 80M.  You should get about 1:1 with that resistor.
- Rig your 80M wire antenna to your box and use your box and the dipper to trim the wire length for a dip.  You won't get a 1:1 SWR, but now you'll have a resonant wire and a resonant box.  The only thing that will be not correct is the turns ratio and from the SWR you'll know what turns ratio you need.
- Assume you need to go up in turns ratio and calculate what you need to get to 1:1.  Re-wind the transformer accordingly.
- Rewinding screwed-up the box's resonant frequency so with a resistor load, re-tune the variable cap so your box is resonant at 80M.  This time use a resistor = 50ohms * (square of turns ratio).
- Test your box with the wire antenna.  It should be about right.  Now the only trick is to figure out what your variable cap is set to so you can get a high voltage cap.
Good luck and keep us posted,
Bob

[gil]

Thanks Bob  :)
Gil.

[Archangel320420]

4.7 K of Impedance on a zepp? I was WAY off. Forget anything I said in this thread and don't follow the antenna dunce Terry. Carry on, boys.

[cockpitbob]

Gil,
I just realized I had a brain-fart with my long post above.  I said in all the EFHW antenna couplers I've done the wire seems like a 4.7K resistance, but I meant to say 2.5K.  Most of the articles I've read say 4.7K but that's not my experience.  I use a 7:1 step-up turns ratio in my transformers and I get very close to 1:1.  Also, the Better QRP tuner uses a 7.3:1 ratio, and it works so well that it must be the right ratio.  I corrected my post above.