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Old 21st Jul 2020, 11:36 pm   #1
crackle
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Default A new long wire aerial

I have been advised to replace my G5RV Junior with a random long wire, of length 124.5 feet. The plan is to use the aerial from 160m to 10m
I obviously cannot make this in a straight line in my smallish garden so I wondered if you guys could comment on the plan attached to install the aerial in a rough open square.
The feed point will be 2/3rd of the way down the garden on a 7m mast and after running around some other masts and a rope around the chimney it will terminate on a 4 foot length of rope attached to the same original start mast.

Comments please.
Thanks
Mike
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Old 21st Jul 2020, 11:44 pm   #2
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Default Re: A new long wire aerial

You'll probably benefit from a more elaborate earth, especially on transmit. You'll find the radiation resistance will dip very low at some frequencies which with my luck would land just on the interesting ones. Also a decent antenna tuner will be needed, an unbalanced one.

A second consideration is how does it compare with what you had in terms of picking up noise? this can be a game-stopper. If you can't hear 'em, you can't work 'em.

David
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Old 21st Jul 2020, 11:48 pm   #3
Phil G4SPZ
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Default Re: A new long wire aerial

Hi Mike,

Personally, I've always tended to avoid aerials with parallel wires in which induced currents will be flowing in opposite directions and could cancel out.
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Old 22nd Jul 2020, 12:24 am   #4
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Default Re: A new long wire aerial

Not sure if it's of any interest but I've recently put up an inverted V for use (primarily) on 80m and my initial impression of that is very favourable.

The last two aerial attempts were horizontal dipoles and both were plagued by RFI on receive and a magloop out-performed them.

I took out two largish trees a while ago (there are just so many damn trees, and an overhead telephone cable and an overhead power cable ). With those trees gone, the V became possible in a position with better separation from the two overhead cables. The noise level on the V is about the same as the magloop, but signal strength is better on the V.

So, whether the reduction in RFI is due to the 'change in polarisation', or the greater distance from the utility cables, or both of these things, is hard to say, but I'm glad I went that way.

B
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Old 22nd Jul 2020, 6:29 am   #5
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Default Re: A new long wire aerial

You might find the ideas for stealth antennas in this article useful, Mike.

https://www.sgcworld.com/Publication...stealthman.pdf

73

Roger/G3VKM
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Old 22nd Jul 2020, 6:45 am   #6
crackle
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Default Re: A new long wire aerial

Quote:
Originally Posted by Phil G4SPZ View Post
Hi Mike,

Personally, I've always tended to avoid aerials with parallel wires in which induced currents will be flowing in opposite directions and could cancel out.
I am definitely not very knowledgeable about antennas but it would always be my first thought that installing an aerial in an array like this would cause problems with opposing waveforms.

Would it be better if it was a closed horizontal loop?

Is it considered maybe better to keep the G5RV J and put this new long wire up as an addition for top band.

Mike

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Old 22nd Jul 2020, 9:06 am   #7
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Default Re: A new long wire aerial

The G5RV is a compromise antenna of some cleverness. It uses an open wire feeder section to rotate the impedance of the off-tune dipole to a good enough match to 75 Ohm coax that a typical valve transmitter of the day could handle by adjustment of its Pi-tank output circuit.

Not perfect, but it got you on 5 of the (then) 6 HF bands.

As an alternative, use whatever length of top you can fit in, Use whatever length of open-wire feeder you need, and have a good balanced ATU at the end of the open feeder. 50 Ohm coax from the ATU to the radio.

A good ATU in this case being a transformer-coupled design like a Z-match. This gives a good break for common-mode signals as well as good balance and some selectivity.

I've got things arranged for open feeder to enter the shack and the tuner (A Decca-KW 109) lives next to the rig. Other people have used coax out to an ATU in a weatherproof box in the garden with remote control of the variables.

With the 'whatever length you can fit' business, you might find a spot in a band where you can't tune-up due to the Z going outside the range of your ATU, but you can then change the length of the top or the feeder to see if you can move it out of the band.

A lot of magazine articles have covered NEC simulations of loads and loads of antennae, but they all put them in rather idealised situations. As few people have an ideal situation, then the results of the sims are simply wasted effort. A more practical route is to have an antenna you can fiddle with until you reach a reasonable compromise.

Designing perfect antennae for ideal situations is relatively easy, but it's in arriving at compromises for real situations that is needed.

Closed horizontal loops become band-specific if driven across a break in them. A whole loop driven against earth? anything could happen, but you'll need a good earth mat.

David
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Old 22nd Jul 2020, 9:26 am   #8
crackle
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Default Re: A new long wire aerial

I have just realised that I can still fit (just) the G5RV Junior in with the mast moved to the new position to support the parallelogram long wire.
My ATU is in the radio shack.
Here is the new plan.
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also uploaded as a PDF to help clarity.


Mike
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Old 22nd Jul 2020, 1:03 pm   #9
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Default Re: A new long wire aerial

If you don't use Louis' design of open feeder length and 75 Ohm coax (he may have used 72 Ohm) Then you don't need to stick to his top length. Just make sure the open feeder comes down from close to centre with the longest top you can have. Stay in open feeder to the ATU in the shack. Build a Z-Match ATU. Don't bother with an unbalanced ATU with a balun on the end. A balun can be made to fo the frequency range and a balun can be made to do a fair impedance range, but they can't do both.

David
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Old 23rd Jul 2020, 12:52 pm   #10
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Arrow Re: A new long wire aerial

Quote:
Originally Posted by Radio Wrangler View Post
A balun can be made to for the frequency range and a balun can be made to do a fair impedance range, but it can't do both.
Agreed; in summary . . . .
Z (comprising of L & C) is a function of f.
Case 1. Specify [Z range]; L and / or C thus determined: get resultant [f range].
Case 2. Specify [f range]; get resultant [Z range]: L and / or C thus determined.
The [f range] and [Z range] in cases 1 & 2 will rarely be same.

Al.

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Old 30th Jul 2020, 11:24 am   #11
Steve G4WCS
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Default Re: A new long wire aerial

128ft loop and 4:1 balun works very well. also for the higher bands Im using a fishing pole with a wire cut for a quarter wave for the band against a quarter wave counterpoise , Ive bungeed the pole to the garage rainwater downpipe and be put up and taken down in a minute
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