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Old 2nd Jul 2018, 6:42 am   #1
its ur aerial
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Default Antenna Loading coil

I am in the process of building a loaded vertical antenna, ( very small garden) to the design of G3RFG, published in the book, HF Antennas for everyone.
The coil is wound using 10 SWG (3.25mm) enamelled wire, which I have found impossible to purchase in small lengths, however I have more than enough 4mm stranded PVC covered copper wire .
Would using this, totally alter the inductance required, or my other option is to purchase 12 SWG and wind a bit more on ?
Any comments much appreciated .
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Old 2nd Jul 2018, 8:02 am   #2
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Default Re: Antenna Loading coil

It won't change the inductance much.

With a bottom coil-loaded vertical the thing to bear in mind is that the max current is at the base and the field from it is wasted in the inductor. Do anything you can to minimise the inductor and get that current up into the radiator where it can, er, radiate.

Get the best vertical height you can. Use a capacity hat. Then see how much inductance you need.

David
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Old 2nd Jul 2018, 12:11 pm   #3
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Default Re: Antenna Loading coil

The antenna is centre loaded, the idea being two different coils for 160/80 and 40/20. I only really want to use it on 80 Metres.
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Old 2nd Jul 2018, 1:46 pm   #4
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Default Re: Antenna Loading coil

Centre loaded is a lot better than bottom loaded. 4mm wire should be OK and the origininal winding details ought to be quite close to right.

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Old 3rd Jul 2018, 7:41 am   #5
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Default Re: Antenna Loading coil

Thanks very much, replies much appreciated. 73`s
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Old 4th Jul 2018, 11:23 am   #6
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Default Re: Antenna Loading coil

Be mindful when using "PVC" insulated wire in this sort of application. I've come across some insulated wire whose insulation was actually not-PVC, and which, when used in a 'close-wound' RF tank circuit suffered enough dielectric-loss to melt the insulation to the point where it coalesced into a black sticky lump.

When winding such loading-coils you should expect to have to do a bit of trial-and-error to get the resonance right: an adjustable top-section or movable capacity-hat helps. On 80/160M you can often find the usable bandwidth is rather narrow with short verticals though !
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Old 4th Jul 2018, 12:58 pm   #7
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Default Re: Antenna Loading coil

Quote:
Originally Posted by G6Tanuki View Post
Be mindful when using "PVC" insulated wire in this sort of application. I've come across some insulated wire whose insulation was actually not-PVC...
Seconded. Also, the stuff that (commonly, in my experience) passes for PVC but isn't is also really susceptible to degrading under UV, even in the UK.

The whole arrangement might last rather longer using enamelled copper wire, much as 4mm is rather awkward to bend!.
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Old 4th Jul 2018, 2:45 pm   #8
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Default Re: Antenna Loading coil

Earth resistance (impedance) is much more important for a short, loaded 'Marconi' antenna, it will be in the order of ohms so a little extra in the coil will not matter much. That said even 16SWG (1.6mm) will be more than good enough for the coil.
 
Old 4th Jul 2018, 5:34 pm   #9
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Default Re: Antenna Loading coil

After reading your opening post I have just ordered the book from the RSGB. I might try the same antenna as I also have small back garden.
Hope you keep us posted with your progress.

John
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Old 5th Jul 2018, 5:09 pm   #10
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Default Re: Antenna Loading coil

The usual problem with short vertical antennas is deciding which to choose between:
1. ground losses if the 'other' connection is a ground spike
2. finding space for a counterpoise, which may need to be a quarterwave in length (although it can be zigzagged around)
You can use a inductive-loaded counterpoise, but then you have two thing to tune.
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Old 9th Jul 2018, 3:40 pm   #11
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Default Re: Antenna Loading coil

Hi another option to experiment with is the Folded Counterpoise. I have a smallish garden but my fence runs from the front of the house so I managed to get a top band version along the side of the house and garden fence.

I followed the one under "K2AV folded counterpoise" search. I did not deviate from the balun feed design or recommendation to use bare wire. Works well with a vertical wire up to a coil hauled up on a pulley near the chimney and top capacity wire. Main top coil is ptfe wire with turns adjusted for resonance with another small coil of a few turns near the balun for a final tweak.


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Old 9th Jul 2018, 4:18 pm   #12
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Default Re: Antenna Loading coil

Folded counterpoises can be useful, as can what I describe as the "hedgehog" counterpoise - a loading-coil fitted direct to the feeder at the feedpoint, with a series of very short 'radials' - fitted wheel-spoke-like as a capacity-hat - on the other end.

The more radials the better - I've used about 10 1-metre-long 6mm aluminium rods on a 3.6MHz loaded-vertical.

The 'hedgehog' counterpoises are used on some commercial antennas too. The Moxon book "HF Antennas for all locations" has details.

Last edited by G6Tanuki; 9th Jul 2018 at 4:23 pm.
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