Originally Posted by KC2ZHY
I'm looking for a BNC antenna tuned to both the UHF-T (450-512) and 700/800 MHz bands for base station mounting. A simple whip is fine; not looking for anything bulky, like a log-periodic, Yagi, or a discone.
Well, a discone or similar (any broad bandwidth) antenna would not be much of a choice in any event if you are intending to monitor specific frequency bands. As Rt169Radio rightly said, a good option would be an antenna tuned/resonant for the freq's you want to monitor - such as the Larsen Tri-Band.
But let me add to that - and I always say this when it comes to choosing an antenna - input filtering is becoming more and more important as airwaves get more and more crowded. You r typical consumer scanner/receiver is big time handicapped when it comes to front-end selection. They are plenty sensitive enough, in fact in many cases too much so with the consequence that they saturate, leaving the receiver to struggle when it comes to tuning into many signals - especially when surrounded by other signals or harmonics of equal or similar strength. As a HAM you'll be all to familiar with that term Signal to Noise ratio (SNR). Noise has increased but front-end selectivity has not kept up in my opinion e.g. I use an old early 1990's WJ 8617B-S1 for a lot of general listening - obtained for around $1000 some years back. To come anywhere near close to the receive performance this old receiver offers I'd have to look at something along the lines of an R9500.
And whats the big advantage with the 8617? Nothing to do with sensitivity - nope, in fact it's sensitivity is actually poor by modern day scanner/receiver standards. In fact, it was to some extent deliberately designed to be "poor", just like it's filter shape and roll-off characteristics are nothing to shout about either. But consider the 8617 against the background of what it was designed to do (i.e. be a highly selective scan & search receiver) and suddenly it all makes sense. Then take the cover off and check out the front-end stages, the IF and filter stages and its no wonder it knocks the socks off any modern-day off-the-shelf consumer scanner/receiver. They cost the DOD 10's of 1000's in their day - today they can picked up on eBay from time to time for around $800 - 1200, God only knows what it would cost to build them nowadays!
Anyhow - back to the point: a good way round the problems of many modern scanner/receivers: frequency specific/resonant antennas, followed by band-pass filters - the loss in signal strength been more than made up for through the decrease in noise the front-end has to deal with. Time and time again I see folk throwing money at good antennas - often too good for the scanner/receiver been used ("Oh, no worries - I'll invest in a real good antenna", they say!!!) - but no consideration or thought is given to the problem that the good antenna then passes on to the scanner/receiver by way of all the extra signal strength and noise it presents the front end, which in many cases is not able to cope, or at best, doesn't come anywhere near to giving the buyer/user the demodulated signal quality it's potentialy capable of, or which is quoted on the spec sheets.
If you are open to DIY options, you could give some thought to building a few resonant dipoles. 5/8ths or similar. Invest your money in some band-pass filters with an antenna switch off ebay, and assuming all is set-up adhering to "good practice" you are almost certainly going to enjoy significantly better Rx performance than the Larsen Tri-band or any similar passive antenna would ever be able to offer.
..... just my 2cents worth.