All test cables and connectors on them exhibit loss,
especially as frequency goes up .....You need to account for these
losses by knowing what they are at the freq you are working on, and
even the service monitor is not perfectly constant across it's
band....Always do a loop check with the barrel first. (use a barrel
that matches the connectors on the device to be tested. That way you
minimize potential unexpected losses..... after you know the expected
loss, you can either bump up the generator to get you to a "zero
setting" or subtract the difference from your device measured
results... If you change the freq, check it again.....until you learn
your setup..... Cables can exhibit differences especially at
wavelength increments of your operating freq or near them... It is
suggested to insert pads in the lines of the service monitor to help
maintain a 50 ohm environment as the can changes impedance as it is tuned.....
If you put the correct pads in you can get your known loss
to about 10 or 20 db increments and make the tests a little easier to
calculate....I generally do not need to use pads on high quality
duplexers as they tend to be more impedance stable, and shift less
while tuning.... At 900 mhz it gets real interesting as cables are
MANY increments of the wavelength and can do really strange things...
In this case a return loss bridge and tuning the proper way works a
little better...
BTW a return loss bridge is what is the "approved" way of
doing this, although the method you are using seems to work for most
of the world quite nicely :-)
Doug
KD8B
At 11:42 AM 3/11/2006, you wrote:
>As it turns out, I have a questionable test cable. Either that or I have an
>attenuator turned on in my test set that I'm not aware of. I'm still
>learning how to use this HP8920. I removed the cables from the cavity and
>connected them together and my measured input was 1.75-2.0 db lower than the
>set output. In other words, the output is set at -15.0 dbm. The unit
>displays the received level at -17.0 dbm.
>
>Someone else mentioned adjusting the coupling loops. On this particular
>cavity, they don't appear to be adjustable. Both connectors are mounted on
>the same plate. There are no screws to loosen to allow the conector/loop
>assembly to rotate.
>
>Jamey Wright
>KD4SIY
>
>-----Original Message-----
>From: [email protected]
>[mailto:[EMAIL PROTECTED] Behalf Of Eric Lemmon
>Sent: Friday, March 10, 2006 11:40 PM
>To: [email protected]
>Subject: RE: [Repeater-Builder] Bandpass Cavity Insertion Loss
>
>
>Jamey,
>
>If your measurement is accurate, 3.2 dB insertion loss is excessive. A
>single bandpass cavity performs best when its insertion loss is between 0.5
>and 1.0 dB. How are you measuring this? If you are using a spectrum
>analyzer with an accurate frequency reference, the preferred method is to
>put a "barrel" (double female) adapter between the two test cables and set
>the reference cursor to zero dB. Then remove the barrel and connect the
>test cables to the cavity. You can then read the insertion loss directly.
>
>If you're using the bandpass filter on the receive side, no more than 1.0 dB
>IL should be used. If you're using the filter on the transmit side to
>eliminate spurs or harmonics, 0.5 dB IL will usually be fine.
>
>73, Eric Lemmon WB6FLY
>
>
>-----Original Message-----
>From: [email protected]
>[mailto:[EMAIL PROTECTED] On Behalf Of Jamey Wright
>Sent: Friday, March 10, 2006 8:12 PM
>To: [email protected]
>Subject: [Repeater-Builder] Bandpass Cavity Insertion Loss
>
>
>I just got through tuning up a Phelps Dodge VHF bandpass cavity. The
>measured loss through this thing is 3.2 dB. I am generating at -10 dBm.
>The level measured on the output is -13.2 dBm. It is a Phelps Dodge VHF
>bandpass cavity but I can't find a model number on it. Is this amount of
>loss normal?
>
>Thanks in advance.
>
>Jamey Wright
>KD4SIY
>
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