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Old 6th Jul 2020, 11:37 am   #1
crackle
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Default Converting a G5RV Junior to a Marconi T type antenna

Hi its me again, still trying to get my head around issues with static on my antenna.

In order to get away from a balanced antenna and the difficulties of effective DC grounding of both halves of the dipole, I am thinking of converting my G5RV Junior (we have small gardens here) to a Marconi T type of antenna.

I was assuming I could use an unun to then effectively ground the antenna.

I am also thinking of adding about a further 6m of wire to the ends of the existing G5RV junior to help increase the overall length. This extra 6m will feed away downwards to a point about 2m above ground and approximately at right angles to the G5RV in a sort of Z shape.
The twin ladder line feeder up to the dipole will be joined at the bottom and this will be the new feed point for the antenna.

My question is; Is this going to make a workable multiband antenna (with my MFJ AMU) and what ratio UnUn should I use to connect the feeder coax to the joined 450 ohm ladder line?

Thanks
Mike
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Old 6th Jul 2020, 12:49 pm   #2
Radio Wrangler
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Default Re: Converting a G5RV Junior to a Marconi T type antenna

I'm afraid it's the opposite of what you're thinking.

It's easy to convert the G5RV itself, just join the open wire feeder pair at the base and drive here. You will need a VERY GOOD ground at this point. Don't bother with an UNUN, it won't do much, but you will need a good, unbalanced, antenna tuner located at the feedpoint, and a buried mesh ground, or a lot of radials

What is on the diagram won't work, the whole point of the Marconi T is that it is driven with respect to ground and there is no ground on the picture. The wound coax choke even isolates it from the shack ground.

As a vertically polarised unbalanced antenna it's likely to be worse at collecting local noise as well.

Sorry,
David
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Old 6th Jul 2020, 12:59 pm   #3
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Default Re: Converting a G5RV Junior to a Marconi T type antenna

I use a half G5RV extended by a few metres to fit the space between two trees. It is then fed by ladder line straight to an SGC auto-tuner and from there by co-ax to the radio. The wires are more or less horizontal about 5m off the ground. This works perfectly well for me both on amateur bands and cadet force frequencies.
If you are worried about static then as has been suggested a high value resistor from each leg to ground will bleed off static and not affect the performance. You can also buy spark gaps specifically for aerial protection.
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Old 6th Jul 2020, 2:40 pm   #4
crackle
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Default Re: Converting a G5RV Junior to a Marconi T type antenna

OK so forget that idea then. the only other option then to get away from a free floating element it seems is to go back to an end fed wire with an unun connecting that to the coax.

I Have currently got a 100k resistor wired across the bottom of the G5RV.

Mike
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