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Author Topic: Random Wire Antennas Can Be Troublesome.  (Read 6164 times)

gil

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Random Wire Antennas Can Be Troublesome.
« on: February 22, 2017, 03:51:31 AM »
Hello,

My survival class this weekend didn't go so well as far as radio was concerned :o, everything else was fun and informative.



What was the problem? Common mode currents, AKA "RF in the shack." Except there was no shack. ::) It was getting dark when I shot my 35.5ft wire up a tree, connected the EARCHI 9:1 UNUN, tuned with my ZM-2 and tried calling on 40m... My key didn't work right, or so I thought... It did work when the ZM-2 was on "Tune." Ah :o The microcontroller in the MTR doesn't like RF... I tried on 30m, 20m, no cigar :-\ No problem, I had my 19ft wire with my 6m fishing pole... Still SOOL. My KX2 isn't so sensitive. There I was in the woods with only one radio and no way to even send my call sign correctly. I even tried looping the RG-174 on a branch to make an RF choke. Total cable length was 25ft or 10m, I can't remember.

Pseudo-random wires can be finicky. I say "Pseudo" because you need to avoid half-wave lengths so your tuner doesn't have to work so hard. I remember trying a random wire and counterpoise on my K1 and feeling needle pricks on my fingers, RF getting to my metal key through the radio...

Using a half-wave wire with an end-fed tuner would obviously work. With a random wire we can add an inline RF choke or use a counterpoise of a specific length determined by trial and error, by spooling the end of that wire, making it essentially shorter or longer. We can also do both... I didn't have either.

Bottom line is, if our wire isn't really random and requires an extra counterpoise wire, why not using a HWEF (Half-Wave End-Fed)? Did I really need multi-band operations? I knew 40m was going to be it after dark...

I was wise to try the MTR with these wires before my trip to the Canary Islands in two weeks because I would not have been able to operate! Lesson learned: Always try your gear configuration before an important event. Which makes me think of all those preppers buying radio equipment to put it in a closet until the SHTF... Unlikely to work at all...

I will make an RF choke and try counterpoise wires, but this reaffirmed my love for the HWEF! Random wires are a band-aid at best.

Gil.

NCGunDude

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #1 on: February 22, 2017, 02:09:28 PM »
I participated in Winter Field Day 2017 this year, and my doublet worked great. My paracord was still on site, and it was only a matter of pulling it up.

I did find out my generator is LOUD! We went with a battery bank charged by a car running on idle.

So, Gil, are you in the photo, and which one is you? I've attached one from our field day.
« Last Edit: February 22, 2017, 02:36:30 PM by NCGunDude »

scarr

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #2 on: February 22, 2017, 04:21:06 PM »
I was wise to try the MTR with these wires before my trip to the Canary Islands in two weeks because I would not have been able to operate! Lesson learned: Always try your gear configuration before an important event. Which makes me think of all those preppers buying radio equipment to put it in a closet until the SHTF... Unlikely to work at all...

I will make an RF choke and try counterpoise wires, but this reaffirmed my love for the HWEF! Random wires are a band-aid at best.

Gil.

I have run into the same problems and thankfully they have occurred close to home. You're absolutely right, people have got to use and test their gear in order to find out what works and what doesn't. Buying gear and putting it away in a closet for SHTF is a dreadful waste.

FWIW I have also run in to occasional problems with EFHW due to coax feed length, but I agree they're extremely versatile and I'm currently piecing together lightweight endfed wires for all HF bands from 80M+. How often I'll have the opportunity to roll out a 40M wire remains to be seen, but I guess it's better to have and not need :)

cockpitbob

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #3 on: February 22, 2017, 09:17:32 PM »
 I've never had problems with an EFHW 8).  Now, my slingshot with fishing reel attached is a different story.  Sometimes it can't handle playing out of the spool at slingshot speeds, snags and either the line breaks or I'm ducking a 1oz lead weight coming back at my head :o

I think Gil is in the back row, 4th from the left with the black hat and green/tan cammo coat.
« Last Edit: February 22, 2017, 09:19:13 PM by cockpitbob »

gil

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #4 on: February 23, 2017, 06:42:39 AM »
Quote
I think Gil is in the back row, 4th from the left with the black hat and green/tan cammo coat.

Correct Bob, 4th if you don't count the girl on the foreground. French army jacket and jeans. :)

I never had problems with HWEF antennas, only random wires. 80m end-feds are a bit of an issue. Finding a 40m tree isn't easy, depending on where you live, and throwing a wire that far up is almost impossible. I tried 135ft of wire horizontally, and even with The Wireman #534 26AWG wire, it gets pretty heavy and sags. You also need good attachment points. 66ft of wire is the longest I care to use on a regular basis. I ought to try an inverted V for 80m.

Using a generator to power a radio is the last thing I would ever think of... If I need one it means I am using the wrong radio... Anything bigger than three 18650 cells or a 5Ah LIPO pack stays home, except the PRC-320 which uses its own batteries (24V 5Ah NiCad). Anything my small 14W solar panel won't charge is an issue.

The next survival class I will take my PRC-320. The MTR or KX2 ($!) are just too fragile in that environment with people walking about wearing boots, carrying logs, etc. Alone you can mind your gear. In a group there is a high-risk something will get damaged, like someone tripping on RG-174 coax. Something I never had to deal with before being a bit of a loner.

The ex-soldiers in the group were interested in the MTR, even though the demo was a flop. I bet they'll like the PRC-320!

The instructor also had us do so knife fighting drills. I took him down Systema style :o I ended-up finishing that portion of the class. It's great when you get an instructor who's ego doesn't prevent him from learning something new.

Gil.

cockpitbob

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #5 on: February 23, 2017, 08:54:42 AM »
I think this spring I'm going to do something I've been intending to do for years.  Make a shortened 80M EFHW antenna. 

I don't see why a coil (or even toroid inductor) can't be put in the EFHW wire to shorten it.  Ideally I'll have a coil with just a few feet of wire for resonance trim that I can put between the EFHW coupler and my 63' wire that I use on 40/20/15.

Lamewolf

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #6 on: February 24, 2017, 10:15:06 AM »
Gil,

Use an FT140-43 core and wind 18 turns of RG174 cox on it for a very effective 1:1 choke to install between the 9:1 and the coax and run a counterpoise wire on the antenna side of the unun.

gil

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #7 on: February 24, 2017, 01:20:58 PM »
Quote
Use an FT140-43 core and wind 18 turns of RG174 cox on it

Good one... I have slip-on ferrites that are just the right diameter for RG-174. If that doesn't work I'll try the toroid.

Gil.

cockpitbob

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #8 on: February 24, 2017, 09:06:45 PM »
Quote
Use an FT140-43 core and wind 18 turns of RG174 cox on it

Good one... I have slip-on ferrites that are just the right diameter for RG-174. If that doesn't work I'll try the toroid.

Gil.
Here's a useful rule of thumb for winding indicators.  The inductance goes up with the square of the number of turns.  For chokes and filters I get as many passes through the hole as I can.  3 turns had 9 times the inductance of 1turn.

gil

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #9 on: February 25, 2017, 04:47:24 AM »
Quote
I don't see why a coil (or even toroid inductor) can't be put in the EFHW wire to shorten it.

Certainly. You might be better off putting the coil in the middle though. Some sort of capacity hat would be great but not practical for a portable antenna...

Gil.

Lamewolf

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #10 on: February 27, 2017, 04:33:32 PM »
Quote
Use an FT140-43 core and wind 18 turns of RG174 cox on it

Good one... I have slip-on ferrites that are just the right diameter for RG-174. If that doesn't work I'll try the toroid.

Gil.

It will take a bunch of slip on beads to do the same as one toroid with those 18 turns on it.  The toroid & 18 turns would be about 6000 ohms @ 80 meters !

cockpitbob

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #11 on: February 27, 2017, 10:04:02 PM »
Quote
I don't see why a coil (or even toroid inductor) can't be put in the EFHW wire to shorten it.

Certainly. You might be better off putting the coil in the middle though. Some sort of capacity hat would be great but not practical for a portable antenna...

Gil.
Ive read that a center loaded vertical will work better than a base loaded one.  It makes sense that would apply to wires too.  Some day I'm going to take the time to figure out EZNEC.  Adding a coil to the base of an end fed wire would be a lot more field expedient than adding it to the middle.  It would be nice to know the difference in performance.

gil

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #12 on: February 28, 2017, 03:08:08 AM »
Right, I don't know either... Also want to try EZNEC some day...

Gil.

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Re: Random Wire Antennas Can Be Troublesome.
« Reply #12 on: February 28, 2017, 03:08:08 AM »