Radio Preppers

General Category => Antennas => Topic started by: gil on April 03, 2013, 12:41:36 AM

Title: An End-Fed Wire Antenna for 80m.
Post by: gil on April 03, 2013, 12:41:36 AM
I love end-fed antennas. My first one was a PAR 40/20/10, and is has become my permanent antenna at home. I have reached 5200 miles on 1.3W and 830 miles with 100mW with it! And numerous other such contacts. I also have a tunable 6-40m I built, and am getting a 15-40m end-fed tuner kit in the mail this week. Why end-feds? Because they work, really well. They are easy to deploy in the field and weigh very little. LNR Precision (bought PAR) has a new trail model by the way: http://www.lnrprecision.com (http://www.lnrprecision.com). But this is about building your own 80m version. I wanted something rugged and simple. An end-fed is simple by design, though the theory isn't so simple. It is an impedance transformer. Here is the schematic I found online (http://www.spirat.com.au/vk5zvs/pic54.htm (http://www.spirat.com.au/vk5zvs/pic54.htm)):

(http://radiopreppers.com/images/80mEFHWT.bmp)

Not much to it really... Let's see with all the components in the box:

(http://radiopreppers.com/images/EF80a.jpg)

The box is a Hammond cast aluminum. The capacitor is a 100pF 3500V doorknob type. I could have used a cheaper ceramic disk type, but you know me by now.. The resistor you see there (black) goes to an isolated ground plug for use with a weather balloon or kite (1.5M 3W). When connected to the earth, it bleeds static electricity to the ground to avoid frying the radio (thousands of volts).

And the finished product:

(http://radiopreppers.com/images/EF80b.jpg)

I though about connecting grounds together via wires, but the box does that well.. The optional counterpoise (we'll see about "optional" later.. (0.05 WL, about 13ft)) binding post (black) is electrically connected to the box, while the antenna wire post (red) of course is isolated. The magnet wire is 18ga. Opinions?

After testing I will pot it with liquid rubber... I did some preliminary testing with a 135ft wire strung around the house and got a 3:1 SWR; just enough to send my call sign and "testing" in CW of course. I very rarely use a microphone.. Higher up the band the SWR goes up, so the antenna is a bit too long. Having many turns at weird angles and a house with all it's wiring around it of course doesn't help. Once stretched outside I expect it to work great. I will post an update.

Now I'll be able to check in CW nets on 80m!

Gil.
Title: Re: An Enf-Fed Wire Antenna for 80m.
Post by: cockpitbob on April 03, 2013, 02:23:57 PM
I love end-feds and this looks nice!  Defenitely QRO.  I just ran some numbers and it is good for well over 100W.  At 100W the losses in the core are about 2.5W (97.5% effeciency) and at 10W are < 1/4W.

Voltages at the output end are high as you know.  I would add insulation between the wires and the metal box.  Magnet wire enamle isn't that durable. 
 
I think you are taking a chance by potting it in rubber (or anything).  Filling the air voids around the wires will increase the capacitance between the wires and core which will reduce the tuned frequency.  You may get the center of its bandwidth just right and then have it shift lower after potting.  I would just tack it down with 4 blobs of RTV.
 
Keep us posted!
Title: Re: An Enf-Fed Wire Antenna for 80m.
Post by: gil on April 03, 2013, 02:47:29 PM
Thanks Bob, great info!

I figured that on 80m, losses can have an impact. Not like on 20m.. The band is noisy and every bit helps.
I might have to rethink my potting plans then.. Maybe something closer, like closed-cell foam.
My idea was to find some kind of mylar for extra insulation between the coil and the case.

Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: cockpitbob on April 03, 2013, 07:38:46 PM
Gil, VK5ZVS has a nice page there, but I'm biased because I like end-feds.

(I hpoe I'm not about to tell you something you've known for a long time.)
Before trimming the wire, consider tuning the transformer.  Though VK5ZVS found 100pF made a perfedt 3.59MHz resonant circuit with the transformer, those cores usually have +/-20% tolerance specifications.  Plus, how you wind the wires on the core will change the inductance and stray capacitances.  If you have an antenna analyzer or o-scope I would put a 2K resistor across the output and sweep the input to see if you get a peak at the frequency you want.  If not, add or remove a turn or crowd or spread the windings.  The antenna will work much better with the coupler resonating at the same frequency as the wire.

Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on April 03, 2013, 08:22:40 PM
Hello Bob,

Yes, thank you. That is why I took the risk of using a fixed capacitor. I was hoping moving the loops on the core closer or wider would give me a little leeway. I have not been able to stretch the wire out yet to test it properly. I could stretch it a few feet off the ground on the street, but I suspect the proximity of the ground would mess things up. So, I will have to wait that I can find a tree to shoot the wire up to, and probably make an inverted V at about 50ft top, the maximum height of trees around here. I might find taller ones when I go camping. I think 50' is reasonable for testing and will probably be the most common height available in the field. It is at my house for that matter.

How do you get the 2K impedance? Wire is 26AWG, 134ft. Copper-clad steel.

Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: cockpitbob on April 03, 2013, 08:59:08 PM
How do you get the 2K impedance? Wire is 26AWG, 134ft. Copper-clad steel.
The purpose of the resistor is to simulate the impedance of a perfectly tuned (purely resistive load) antenna, but its value doesn't matter too much for finding resonance.
 
Opinions vary on what the feedpoint impedance of an EFHW antenna is.  I've seen/heard of everything from 1.5Kohms to 5Kohms.
 
For the 40M - 15M ones I've done I find the antenna's impedance is around 2.5K so I use a 7:1 turns ratio. Zooming in on VK5ZVS's pictures it looks like he used 2 resistors adding to 5K  for the variable capacitor test setup, so maybe he's expecting 5K for the 80M wire.
 
The favorite one I made uses a broad-band transformer and no capacitor.  With a 65' 3" piece of insulated wire it's a great 100W antenna on 40M and 20M, a good one on 15M and there are useable portions of 10M.  It's pretty small and there's no tuner or anything to adjust when switching bands ;D .  As soon as I get the taxes done I'll see if I can't write it up and post it.
 
ETA:  I take that back.  As soon as I get the taxes done I'm going to shrink it and make a tiny 10W version. 
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on April 26, 2013, 06:20:27 PM
Well, looks like I'm on 80m now, kinda... I shot up a wire over the house and the trees, and ended up with a half-wave wire in a shape I would call a "shortened inverted U." SWR is 2.4:1 on 3560. Not great, but useable. I suspect it would be much better with a straight wire. I used a 13ft. counterpoise. No tuner. So, who's going to call me on 80 tonight, CW?

Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: cockpitbob on April 26, 2013, 06:35:22 PM
I have a 180' long wire in an inverted-L and a tuner that forces it to do 80M pretty well.  What time & freq?
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on April 26, 2013, 06:43:28 PM
3560KHz, any time tonight, I am on now :-)

Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: WA4STO on April 26, 2013, 08:36:35 PM
Heh, that's funny; I was on 3562 which is where the TENth Region traffic net meets.

Millions of freqs but small world!  :)

73

Luck
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on April 26, 2013, 09:00:31 PM
Quote
Heh, that's funny; I was on 3562 which is where the TENth Region traffic net meets.

Now I can look into the NTS...

I am monitoring 3560...

Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: cockpitbob on April 26, 2013, 09:42:54 PM
Hi Gil,
Sorry I stood you up.  I ended up working at my desk until 7:45, then dinner and family.

Now I'm printing the VE manual because our club is giving a license-in-a-day class tomorrow and with my brand new VE badge I'm going to help.  I figured while the students are studying I'll review the manual so I'll actually know what I'm doing.
--Bob
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on April 26, 2013, 09:44:20 PM
No problem Bob. I have some SWR issues anyway.. Probably not a good idea for me to transmit at this time..

Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: WA4STO on April 27, 2013, 12:20:08 PM

Now I can look into the NTS...


Gil.

Since you're into CW, you'd be SUPER welcomed into the National Traffic System.  Florida is just a hotbed of traffic handlers as is the entire 4th Region (Virginia down through Florida).

My recommendation would be to listen in to the:

 1900             QFN/E   All Florida CW Traffic Net (Early) 3651

or the

 2000             QFNS    All Florida Slow Speed CW Tfc Net  3715

but that info is 8 years old now, so it might not be accurate, time/freq - wise.  Still, it's worth a shot. If you pick up something (strange Q-signals or whatever) that you don't understand, just ask here.

73

Luck
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on April 27, 2013, 12:27:43 PM
Thanks Luck, I will definitely listen to those frequencies..

Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: KK0G on April 27, 2013, 08:38:26 PM
So what was the end fed tuner kit you ordered?
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on April 27, 2013, 09:13:40 PM
Quote
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.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: KK0G on April 27, 2013, 11:02:20 PM
Quote
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.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on April 27, 2013, 11:31:08 PM
Quote
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.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on September 30, 2013, 12:56:46 AM
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.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: Archangel320420 on September 30, 2013, 01:20:21 AM
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?
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: Archangel320420 on September 30, 2013, 01:22:26 AM
I  meant... The SWR should be high in this situation.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on September 30, 2013, 01:24:13 AM
Quote
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 (http://www.aa5tb.com/efha_wrk.html)
The site had great articles: http://www.aa5tb.com (http://www.aa5tb.com)

Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: Archangel320420 on September 30, 2013, 01:41:40 AM
WHOOOOAAAA   Everything you ever wanted know and not know about end-fed antennas  :)


http://www.eham.net/ehamforum/smf/index.php?topic=65832.0
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: Archangel320420 on September 30, 2013, 01:56:24 AM
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.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on September 30, 2013, 03:54:33 AM
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:

PriSecRatioFreq
30310:15.15
2939.6:15.15
2839.3:15.3
2739:15.45
27213.5:15.32

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 (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.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: cockpitbob on September 30, 2013, 09:06:40 AM
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
 


 
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on September 30, 2013, 12:51:18 PM
Thanks Bob  :)
Gil.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: Archangel320420 on September 30, 2013, 07:20:15 PM
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.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: cockpitbob on October 03, 2013, 06:46:26 PM
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.
Title: Re: An End-Fed Wire Antenna for 80m.
Post by: gil on October 03, 2013, 07:05:10 PM
Interesting. I got a reply from Steph, the original author, and she said to use 4.2K...
I'll get back to it when the fancy hots me, which could be any time!  ;)

Gil.
Title: Re: An Enf-Fed Wire Antenna for 80m.
Post by: Lamewolf on October 15, 2013, 03:01:57 PM
Thanks Bob, great info!

I figured that on 80m, losses can have an impact. Not like on 20m.. The band is noisy and every bit helps.
I might have to rethink my potting plans then.. Maybe something closer, like closed-cell foam.
My idea was to find some kind of mylar for extra insulation between the coil and the case.

Gil.

Just wrap the coil in teflon pipe tape, its a good insulator and holds up real well to high voltage !