Hi Dave, G3YXM,
Just to finish our brief discussion yesterday on the Lowfer's Net, my reason
for trying a transmitting loop first is so that I can operate on both HF and
LF without having to put up two antennas.
The problem is that the garden is rather short and so I have had to bend the
ends of the G5RV downwards. The top is 64 feet and so each end descends 19
feet. If I were to use the antenna as a top-loaded vertical, I would
probably have to chop off these two descenders, which would probably ruin
its low-angle radiation for HF. The ends come down to about 7 feet above
ground and so it is easy to attach another 64 feet or so of wire a couple of
feet above ground to complete the loop. It is then fed via the vertical
feeder to the centre of the top. At the bottom of the feeder will be a
water-proof box containing alternative matching to heavy gauge 50 ohm coax
(Mr Westlakes best) for either HF or LF. To operate on HF, I would have to
disconnect the lower part of the loop and switch to an HF balun. For LF
reconnect the lower loop and switch to a Loop Tuning Unit as described in
Steve Rawlings home page. He has kindly given me pointers to sources for
components.
Ideally the switching might be made using high voltage relays as in the
article on Transmitting loops on page 2.25 of the LF Source Book, but
initially I would do it manually.
The principal problem I have found during initial tests, is that the DC
resistance of 300 ohm feeder (best quality air spaced) is 0.7 Ohm for 34
feet - very high! So I am making my own out of 50/.25 wire with 3inch
spreaders, which will reduce it to about a tenth of this figure. By the way
can anyone complete the graph of RF resistance against frequency from Les
Moxon's book (HF antennas for all Locations fig 3.10) for the bit of
interest between 1 MHz and 0Hz. The graphs are shown as straight lines, with
the resistance increasing with decreasing frequency, but it must turn over
at some point in order to come down to the DC resistance at 0Hz.
This must be a problem others have faced, how to use a single antenna for
both HF and LF - but I haven't seen any discussion about it.
Nice to meet you on the air for the first time, all be it not on 136 yet,
but I have heard you on that band many times, including twice on Sunday,
when you were S8 here in Reading despite the fact that the RX loop is
East-West at the moment.
73s
John, G4CNN, IO91ML
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