>Thanks a lot for the corrections.

>The fairy tale helped me, though -
>I had super signal through from antenna 1 to antenna 2, but the
>other way - NIL.

>I then cut 17 cm off the cable and voila - it worked.
>Any explanations ?

The business of making the cable a complete number of half wavelengths long is a
not unreasonable practice unless you have matching networks that you can
specifically tune to take out any impedance mismatch. There will always be some
mismatch between the feed wire and the aerial. If the cable is an odd number of
quarter wavelengths long then this can represent a very high impedance at the
transmitter (or receiver) end, resulting in considerable losses. If the cable is
a complete number of half wavelengths long then the impedance will be very close
to that at the aerial, which will almost be guaranteed to be a closer match than
add an odd quarter wavelength different. At VHF and UHF this does become of some
importance as the output networks at the transmitter are not normally tuneable
to tune out any mismatch.

At HF and lower, because the networks are designed for lower bandwidths and
typically have components which are of a type that can be varied, then the
feeder length is less critical. Also at HF it is common to have a tuneable
component at the aerial end to get a closer match between the aerial and feeder.
At VHF there may be an attempt at matching the aerial to the feeder, but this is
generally a broadband match, which is not often tuneable, and is only done when
the aerial is far off a 1/4 or 5/8 wavelength long. Do remember that an aerial
does have a reactive impedance that also has to be tuned out by the matching
network for best energy transfer, as well as the resistive portion of the
impedance.

In your case it sounds like the feeder was close to an odd number of quarter
wavelengths long, and so the transmitter was working into what it saw as an open
circuit (or significantly high impedance), and changing the feeder length
brought it close to a half wave multiple which allowed to transmit a significant
proportion of its energy, and so radiate a worthwhile signal strength.

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body