----- Original Message -----
From: "hamilton mal" <[email protected]>
HI Jim.
This looks like a nice tuning aid and judging by your signal it must be
working, the only other signal from the South that moves my S meter is
G3YXM, the others could certainly benefit from your meter. I wonder how it
works with mains electric cable instead of coax to the antenna. Someone
recently mentioned using this cable, and it seems popular on the South
Coast. I have always designed my TX/RX system to use 50 ohms, that way I
know exactly how to measure the performance and it reflects on signals
radiated and received from this QTH.
As far as I am aware I am the only one who has confessed to using electrical
mains cable as transmission line so I guess the above is aimed at me.
The distance from my LF transmitter to the primary loading and tuning coil
is about two metres. This makes the length of my "transmission line" about
.0009 wavelengths. To put that into perspective that is about 20mm on 14MHz.
The impedance of the transmission system is unimportant, although there are
certain considerations. For example, the distance from the Decca transmitter
site to the antenna loading/tuning building was often a couple of km so a
higher impedance system is chosen to reduce I^2 R losses.
Early amateur home brew transmitters (and some commercial) using pye tanks
were often designed with a nominal 75ohms output due to the availability of
cheap television coax.
50ohm systems were chosen by equipment, test gear, coax cable and antenna
manufacturers in the interests of standardisation - there is nothing 'magic'
about 50ohms systems.
There is no point in having an interim 50ohms section between a transmitter
(that is other than 50ohms) and the antenna matching/loading system unless
you wish to use it with a piece of existing or proposed 50ohm test
equipment.
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
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