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Author Topic: Random wire antennas and tuners  (Read 214 times)

CPR

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Random wire antennas and tuners
« on: May 17, 2018, 09:22:06 AM »
I got myself a Z Match Multi Tuner from QRP Guys. With 35$ its quite cheap, but winding the toroids is a pain in the butt.
First, a question about the functionality before I head over to the random wire antennas. I hope that's okay and won't be classified as offtopic.

Yesterday I tuned my triband vertical and decided to try out the tuner with the vertical to see if it does something.
When I switch to "TUNE" I can get the SWR down to 1.1. As soon I switch over to the "OPERATE" position the SWR jumps over 10.. I was using my Mini60s attached to the tuner going then to the antenna. I thought maybe the power of the mini60s is to weak to get a reading on operate, but I am not sure and pretty clueless if the tuner is maybe malfunctioning.

Another question is the random wire, assuming the tuner works, I thought about attaching the random wire and use it as an endfed antenna! Would be the cheap option for now, as my favourite endfed multiband is the mini from HyEndfed and with over 100 it's clearly over my budget at the moment.
Anyway, I found a good website about random wires: http://udel.edu/~mm/ham/randomWire/

Does anyone have any experience on the "best" length given: 
Quote
A standard recommendation (see QST, March 1936, p. 32, "An Unorthodox Antenna") is an 84' long end fed and a 17' long counterpoise (6.5' for 20m)

How do you layout the counterpoise at a random wire? Do you have to adjust the length of the counterpoise depending on the band tuned or does the tuner take over that part too? I don't understand what they mean with this 6.5' for 20m in the bracket.

Best,
CPR


gil

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Re: Random wire antennas and tuners
« Reply #1 on: May 17, 2018, 02:08:26 PM »
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I am not sure and pretty clueless if the tuner is maybe malfunctioning.

Probably. I would re-check your wiring, there has to be a mistake in there somewhere...

Gil.

CPR

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Re: Random wire antennas and tuners
« Reply #2 on: May 18, 2018, 06:29:40 AM »
No, it's all like it should be. I contacted QRP guys about that issue but haven't got an answer yet. I should have got that MFJ971 for portable tuning, IMO that tuner kit is garbage.

Andywragg

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Re: Random wire antennas and tuners
« Reply #3 on: May 18, 2018, 05:12:37 PM »
I saw the other post earlier during my lunch break.

I'm assuming you haven't done this already...

The first thing I would do is take the wires off the "bobbin" PCB and using a multimeter  (set to check continuity) check the continuity between the BNC Outer and counterpoise bolt, and the BNC inner and the main radiator wire bolt. Also check for shorts across the BNC outer and inner. Do this in ALL switch positions, as you may have a faulty switch or you could have a dry joint on one of the toroids. I normally do these checks as part of the build.

Once you have verified the continuity of the PCB check the continuity of the wires, mainly the main radiator wire, but also the 4 counterpoise wires. Do the same check with your Coax feeder (Inner/outer/short test).

Lastly check your radio and tuner with a dummy load.

I scratch built mine and its pretty damned good for a homebrew antenna. No tuner required on any of the bands. And all I did was copy the QRP Guys schematic in the manual. But until I'd got the meter beeping on everything but the short tests I didn't hook it up to the radio.

Hope this helps

Andy

Once a 'STAB in the Dark'
M6UAW

CPR

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Re: Random wire antennas and tuners
« Reply #4 on: May 19, 2018, 02:45:52 AM »
Thanks for the hints andy. Will do that.
Regarding the bad swr on the verical triband. It was caused from the radials not touching the ground. I staked them out so before, was a mistake.

Via TapaTalk


CPR

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Re: Random wire antennas and tuners
« Reply #5 on: May 22, 2018, 06:05:20 AM »
I have cut and built the random wire for my tuner. I can get 1.2 SWR across the bands when my tuner is on the tune position but as soon I hit the OPER position the SWR skyrockets > 10. I am clueless. Anyway, usually, you attach the wire directly to the tuner with the ground wire. To avoid hanging your tuner directly to the wire, you get an 9:1 unun to feed the random wire from tuner to coax, is that correct?



gil

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Re: Random wire antennas and tuners
« Reply #6 on: May 22, 2018, 06:48:13 AM »
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To avoid hanging your tuner directly to the wire, you get an 9:1 unun to feed the random wire from tuner to coax, is that correct?

It does help in that regard, but the main goal of using a 9:1 UNUN is to lower the impedance of the random wire so that the tuner can tune it. Some tuners are good enough to tune a random wire without a UNUN, but not all of them. I also like the UNUN option because it lowers the impedance for the coax, and thus lowers cable losses. The best of course would be a Tuner/UNUN combination; QRPGuys have one.

I would suggest contacting QRPGuys about your tuner, something is definitely not right there...

Gil.

CPR

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Re: Random wire antennas and tuners
« Reply #7 on: May 22, 2018, 07:05:06 AM »
It does help in that regard, but the main goal of using a 9:1 UNUN is to lower the impedance of the random wire so that the tuner can tune it. Some tuners are good enough to tune a random wire without a UNUN, but not all of them. I also like the UNUN option because it lowers the impedance for the coax, and thus lowers cable losses. The best of course would be a Tuner/UNUN combination; QRPGuys have one.

I would suggest contacting QRPGuys about your tuner, something is definitely not right there...

Gil.

Okay. In that case I'll order myself a thoroid, get some wires and build an UNUN. I contacted QRP guys but not getting an answer from them. I have a spare polyvaricon variable capacitor that I am going to use to exchange theirs, because it seems the inner insulation between the metal plates is damaged. Maybe that's the issue, I dont know.

gil

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Re: Random wire antennas and tuners
« Reply #8 on: May 22, 2018, 07:06:53 AM »
What's weird is that it seems to work in tune mode... Look at the circuit diagram and what happens when you flip that switch...

Gil.

CPR

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Re: Random wire antennas and tuners
« Reply #9 on: May 22, 2018, 07:20:11 AM »
What's weird is that it seems to work in tune mode... Look at the circuit diagram and what happens when you flip that switch...

Gil.

It just bypasses the whole SWR indication circuit.

Quote
The tuner is rated at 5W CW, 10 watts PEP max. and incorporates the N7VE LED absorption bridge
circuit for sensing SWR. In the TUNE position, you cannot damage your transmitter caused by a high
SWR. The worst your transmitter is looking at is a maximum of 2:1 SWR in the TUNE position.
« Last Edit: May 22, 2018, 07:21:46 AM by CPR »