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LF: Antenna experiments

To: [email protected]
Subject: LF: Antenna experiments
From: "John Sexton" <[email protected]>
Date: Mon, 04 Mar 2002 08:38:39 -0800 (PST)
Reply-to: [email protected]
Sender: <[email protected]>
Hi Guys,
Having a rather unfavourable situation on the side of a hill topped with 30 
metre trees, I have been struggling for some time to improve my antenna. 
According to the deeds the back garden has a slope of 34 degrees, but it looks 
more like 45 degrees to me. The garden is 25 metres square and at the end, one 
is on the same level as the house roof. I have a 7 metre mast at the high end 
and attach a wire to the house ridge at the other about 8 metres above ground, 
but because of the slope of the land this wire is on average only about 6 
metres above ground and is limited to 20 metres in length because of proximity 
effects from trees and house. A semi-vertical wire hangs from roughly the 
middle down to the loading coil some metres away from the house.
Recently I bought a Funktechnik 10m fibre glass mast from Sycom, with the idea 
of propping up the middle, but the mast tapers from 51mm at the bottom to 2mm 
at the top, and despite all my efforts to provide a frictionless loop at the 
top to allow the horizontal wire to pass through, the whole thing did a 
vertical u-turn.
Realising this wasn't going to work, I came up with the idea to allow the new 
mast to stand alongside the semi-horizontal wire at the highest point in the 
garden consistent with being at least 6 metres away from the trees, and to run 
a wire up the new mast from the level of the passing horizontal wire and to 
join these two at that point, with a loose loop to allow for movement during 
the frequent gales that we get here. Rather than just run a 4 metre wire up to 
the top, I wound 30 metres of wire over these 4 metres to form a helix. In the 
initial version I put the top section of a G whip at the very top, but this 
burnt out and fell off. Interestingly it was the fibre glass that burnt, it is 
black and perhaps the filler is carbon??
I found that these 4 metres are contributing about 60pF to the capacity, so 
more than 4 metres of straight wire (20pF) but less than 30 metres of wire 
(150pF). All figs approx.
Results are a little unclear at the moment, Jim M0BMU measured my sigs at 10 
uV/metre at 63 km. Many thanks Jim. That is exactly S8, and gives an EIRP of 
13mW and an ERP of 8mW. Despite the changes the resistance at resonance is 
unchanged at 93 ohms and with 200 watts out, I calculated a Radiation 
Resistance of 6mOhm. However Dave G3YXM thought my sigs unchanged. The proof of 
the pudding is as they say in the eating, but it looks to me as though I have 
gained about 1 S point.
When the weather gets a bit better and my back stops aching I plan to increase 
the amount of wire in the helix and reduce the straight bit below, maybe also 
to increase the diameter of the helix. Also thinking how to add some top 
loading, perhaps some very fine wire with little weight, but it might break too 
easily.
I would be grateful for any reports so that I can assess the success or failure 
of this approach, but for a confined space this seems to be a useful 
arrangement.
Apologies if this email is too long.
73, John, G4CNN, having fun.



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