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From: Repeater-Builder@yahoogroups.com
[mailto:repeater-buil...@yahoogroups.com] On Behalf Of Nate Duehr
Sent: Tuesday, March 09, 2010 7:24 PM
To: Repeater-Builder@yahoogroups.com
Subject: Re: [Repeater-Builder] Dual receivers on one antenna for RX only
site
On 3/9/2010 4:53 PM, Gary Schafer wrote:
Without the proper length cables between the cavities and the antenna T
connector both UHF and VHF signals will be attenuated depending on the luck
of the cable length.
What technical reason causes this?
Nate
Hi Nate,
A UHF pass band cavity for example will pass only a UHF frequency that it is
tuned for. On frequency signals coming into it will see 50 ohms. Off
frequency signals will see a short circuit and will be greatly attenuated.
The input loop of the cavity (as well as the output loop) looks like a short
circuit at all but the tuned frequency. So anything that happens to be in
parallel with the loop will also see the short circuit if the frequency is
not that to which the cavity is tuned to.
So if you had a half wave length cable between the cavity and your T
connector, then the short circuit at the cavity (off frequency short) would
also look like a short circuit at the T connector. No problem for the UHF
signal as that frequency sees 50 ohms at the T. but any other frequency sees
a short circuit at the T and would be attenuated there.
Now if that cable was a quarter wave length instead of a half wave length,
the short circuit (off frequency short) would be transformed to an open
circuit at the T connector. That would allow all other frequencies to be
present with no attenuation at the T.
If you used a random length of cable here, you may be ok and you may not be
depending on how far away from a quarter wave length the cable happened to
be.
This is exactly how a duplexer works. The cables between the T and each
cavity set is a quarter wave length at the opposite frequency for which the
cavity is tuned to. The quarter wave length cable connected to the T always
wants to see a short at the other end at the frequency that it does not want
to pass, as the quarter wave length transforms the short to a open which
does not load down the other side of the circuit..
With close spaced duplexers sometimes the two cables may be very close in
length or the same as the cable is not near as high a Q as the cavity is.
73
Gary K4FMX
