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Re: LF: Weekend activity

To: [email protected]
Subject: Re: LF: Weekend activity
From: "James Moritz" <[email protected]>
Date: Tue, 18 Dec 2001 18:39:31 +0000
In-reply-to: <[email protected]>
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
At 19:19 17/12/2001 -0500, you wrote:
Hi Jim I am planning my transmitting station . Could you send me details of your
ant?
    Is it end Fed/ Is it a vertical? What is ground system
I could set up a long sloping wire 4 meters high at fed end 13 mtr high
at far end es 80 meters long.  How does that sound?

Dear John,

In the daytime, my ant is a simple inverted L about 8.5m high on average and with a 40m long single top wire. At night, when the neighbors can't see, I prop it up in the middle with a 13m pole to produce an inverted V 13m high in the middle, 10m at the ends. This increases the radiated signal by about 4dB. The downlead is still connected to the end of the V; I don't think it matters terribly much where it is fed. The loading coil lives in a shelter with a timber frame covered in polythene sheeting at the bottom end of the downlead. After some experiments, I found that having the loading coil outside several yards from the house reduced the losses by a dB or two. The ground system is currently about 8 x I metre long ground rods scattered around the feed end, plus the house wiring and plumbing. More than about 6 rods seems to make very little difference - the ground here is wet clay, and seems to be fairly easy to make a good connection to; probably the same would not be true of sand or rocky soils.

The desirable objective with LF antennas is to get the high current parts as high up as possible - a concern with your proposed wire the part that is 4m high will effectively rob current from the higher part of the wire, and will increase losses while reducing the radiated signal. A couple of suggestions are:

-Use some kind of temporary support at the 4m end - even using a simple length of timber as a temporary way to increase the height by a few meters will help a lot.

-Have the downlead in the middle of the span, replace the 4m high end of the wire with rope, and have a shorter top loading wire section from the middle of the span to the higher support. This will increase the effective height, and in theory should give about 2-3dB more radiated signal, although you will need a bigger loading coil to make up for the reduced capacitance, and the feed point will be in a less convenient place, so you would need to put the loading coil in a box with some sort of remote tuning.

Or do both of course - Having said all that, my first LF antenna was only about 5m high on average, and using 100W I was able to get around Europe on QRSS fairly easily, so it would definitely be worth your going ahead if you end up using the wire as originally suggested. Good luck, I'm sure you will have plenty of people looking for your signal.

Cheers, Jim Moritz
73 de M0BMU




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