----- Original Message -----
From: "g3ldo" <[email protected]>
To: <[email protected]>
Sent: Thursday, May 01, 2003 10:30 PM
Subject: LF: Mains Cable
----- 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.
That is exactly the reason for using either 50 or 75 ohm transmission line,
so that existing station test equipment can be used to test and monitor what
is going on, otherwise its guesswork, like tuning for max smoke, which leads
to buckets of exploded FETS.
For those using the old audio amps, the preferred method is to couple the
4ohm audio output via a 10 mfd (non electrolitic) capacitor to a transformer
designed for 4/50ohms then 50 ohm coax to the antenna loading coil, which is
resonated at the appropriate frequency and impedence matched to the 50 ohm
feed cable. This way is it easy to monitor what is going on with with the
normal shack test equipment like Dummy load, SWR meter etc and is very
economical with FETS , you should not really damage any because you can see
danger coming and avoid it.
G3KEV
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
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