Jeff, I completely agree with your conclusions. The calculations of CommShop, while remarkably close to reality for 2m, 220, and 440 applications, are misleading at 6m and lower frequencies.
73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Jeff DePolo Sent: Friday, January 22, 2010 11:59 AM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] 6 Meter Repeater spacing (no duplexers) > I plugged the same values as before into CommShop, but this > time with a 1 > MHz split, and the result is about 85 dB isolation, which can > be met with > about 7.75 miles of horizontal separation. <snip> > The definitive method for testing whether your transmitter is causing > desense to your receiver is to radiate a weak signal to your > receive antenna > that results in a 12 dB SINAD reading on your service monitor > with your > transmitter off. Then, energize your transmitter. If the SINAD reading > drops, you have desense. As others have pointed out, the use > of one or more > bandpass cavities on either or both ends may greatly reduce > or eliminate > desense. You may be pleasantly surprised. > > 73, Eric Lemmon WB6FLY On 6m, it's pretty hard to find a site with a noise floor low enough that you will ever realize the "bench" sensitivity of the receiver. More often than not, the math says that you will have desense due to insufficient transmitter noise supression, but in the real world you never notice it because that Tx noise is hidden in the ambient noise floor. FWIW, I have a split-site 6m repeater (actually, it's off the air currently) on two high rise buildings in Philadelphia. Tx and Rx are Mastr II, TPO is 110 watts, antennas were originally HyGain V6R's but are being replaced by Kreco "co-plane". The two rooftops are equal height, and are about 150 yards apart. There is no additional filtering at either the Tx or Rx. No desense. However, the difference between the receiver's "bench" sensitivity and the effective sensitivity at the site is about 12 dB (i.e. 12 dB SINAD is about -106 dBm) when connected to an antenna. Quite often I see effective sensitivity on 6m as being up in the microvolt range, so you might want to plug in higher sensitivity values for the receiver spec (rather than 0.3 uV) when doing the math. FWIW, GE's duplex isolation curves show that you need 59 dB of Tx noise supression and 45 dB of carrier supression for a Mastr II lowband station at 50 watts and 0.2 uV, 1 MHz split. --- Jeff WN3A
