I'm moving a 162 MHz (marine band ship tracker - AIS) receiver to within a
couple feet of its quarter wave ground plane antenna.


Is there a certain cable length I should use to tune for maximum gain?

--
Robert Haston
Satellite Beach, FL

On Sat, 22 Jul 2006 23:03:29 GMT, "Robert Haston"
wrote:
I'm moving a 162 MHz (marine band ship tracker - AIS) receiver to within a
couple feet of its quarter wave ground plane antenna.
Is there a certain cable length I should use to tune for maximum gain?

Hi Robert,

The shortest line would have the most gain. However, in all
practicality, if you are not talking a great distance, it doesn't
really matter much for receive from an antenna suited for the band you
are listening to.

73's
Richard Clark, KB7QHC

No.

Robert Haston wrote:

I'm moving a 162 MHz (marine band ship tracker - AIS) receiver to within a
couple feet of its quarter wave ground plane antenna.


Is there a certain cable length I should use to tune for maximum gain?

On Sat, 22 Jul 2006 23:03:29 GMT, "Robert Haston"
wrote:

I'm moving a 162 MHz (marine band ship tracker - AIS) receiver to within a
couple feet of its quarter wave ground plane antenna.


Is there a certain cable length I should use to tune for maximum gain?


There are some very common misconceptions around regarding
cable length impedance matching.

Ideally your antenna has 50 ohms, the cable has 50 ohms,
the transmitter has 50 and there is perfect matching all the time,
at any cable length.

Side note:
Did you note I left out the receiver?
Does anybody know the input impedance of his receivers in use?
How would you measure the input impedance?

Back to the cable lenght, may I spill a few words.
One may use the feed cable to match a non 50 Ohm antenna
to the 50 ohm transmitter, but that works sufficiently only for
a very narrow frequency range. Some people do it on shortwave
and they switch different lenght cable loops with a relais
to cover an entire SW band. Works fine, if no antenna tuner
is available. On the UKW bands this is rather unusual,
and normally not necessary. Go and tweak the antenna
lenght and the radials to optimum SWR. The cable tuning
method may fail with broadband antennas, of course.

w.

On Tue, 25 Jul 2006 15:38:04 +0200, Helmut Wabnig
*_.-_- wrote:

Does anybody know the input impedance of his receivers in use?

Hi Helmut,

Stand-alone receivers may differ, but in transceivers, the receive
path, up to a point, is identical to the transmit path. As such, a 50
Ohm design is universal. Now, if there is any divergence from that
value, you must expect that signal power is being reflected away from
the detector. Certainly, any high Z active amplifier may be used, but
in an untuned front end, this is hardly a virtue.

Stand-alone receivers, like the R-390, can have higher Z inputs with
tuned front ends. The R-390 includes what is called an antenna
trimmer to further optimize the input. It also has two distinct
antenna inputs, balanced and unbalanced.

How would you measure the input impedance?

This takes a special signal generator with a known output Z. You
simply place a series variable resistor in line, and watch the
receiver's signal meter as you vary the resistance until half the
voltage is observed. The input Z is the sum of the signal generator's
output Z and the variable resistor's value. Take warning that a
variable resistor's value is going to be the next difficult thing to
determine. It is not simply a matter of measuring its value (or
rotational angle) with a VOM. However, absolute accuracy is rarely
needed in this regard and a VOM reading will be suitably close.

73's
Richard Clark, KB7QHC

ORIGINAL MESSAGE:

On Tue, 25 Jul 2006 15:38:04 +0200, Helmut Wabnig
*_.-_- wrote:

Side note:
Did you note I left out the receiver?
Does anybody know the input impedance of his receivers in use?
How would you measure the input impedance?

------------ REPLY SEPARATOR ------------

I would think a MFJ SWR analyzer would do it. Haven't tried it, but
can't think of a reason why it wouldn't work.

Bill, W6WRT

On Tue, 25 Jul 2006 11:12:44 -0700, Bill Turner
wrote:

ORIGINAL MESSAGE:

On Tue, 25 Jul 2006 15:38:04 +0200, Helmut Wabnig
*_.-_- wrote:

Side note:
Did you note I left out the receiver?
Does anybody know the input impedance of his receivers in use?
How would you measure the input impedance?

------------ REPLY SEPARATOR ------------

I would think a MFJ SWR analyzer would do it. Haven't tried it, but
can't think of a reason why it wouldn't work.

Hi Bill,

Good suggestion.

73's
Richard Clark, KB7QHC