VHF Quantar- how far off can the duplexer be?

[Equinox]

Have a local SO that wants to have their Quantar programmed with seperate digital and analog channels. Currently, the digital TX freq (155.5875 new freq) is far enough from the analog TX freq (155.700 old freq) that the PA goes into low power mode when TXing on the digital channel. I've tried to convince the guy to just run Mixed Mode on their old freq, but since he went to the trouble to get a new freq for digital (and already had it programmed into his mobiles) he's hell bent on using it. I called MOTO and was told by the clone who answered that I should be able to tune the duplexer to split the difference between the two freqs and be okay. I have my doubts. Just how far off can the duplexer TX tuning be before it causes a problem?

[wavetar]

Typical pass/reject filters have a sharp cut-off at around 200KHz. Your frequencies are 287.5KHz apart, so tuning the duplexer for in-between will give you approx 144KHz from the tuned frequency to either of your active freqs. This should allow it to work without setting off the PA cut-back circuitry on either channel, however there will be several dB of degradation compared to having the duplexer tuned for the exact frequency.

Todd

[xmo]

As Todd indicated, this is a duplexer issue, not a Quantar issue.

It really depends on the frequencies [all of them- Tx & Rx] plus the specs of your particular duplexer. Duplexers are chosen to meet specific requirements of Tx to Rx frequency separation, power level, etc.

If you have the duplexer that Motorola ships, look in the Quantar service manual behind the duplexer tab where you will find performance specifications. Compare all of your frequency requirements to the spacing capabilities listed in the table to determine if this is practical.

The ideal solution is a second Quantar for the second freq - or maybe an MTR for the analog freq and keep the Quantar for digital. Mixed mode conventional is just too confusing for users.

[tvsjr]

The ideal solution is a second Quantar for the second freq - or maybe an MTR for the analog freq and keep the Quantar for digital. Mixed mode conventional is just too confusing for users.

Or a TKR750...

[Equinox]

As Todd indicated, this is a duplexer issue, not a Quantar issue.

It really depends on the frequencies [all of them- Tx & Rx] plus the specs of your particular duplexer. Duplexers are chosen to meet specific requirements of Tx to Rx frequency separation, power level, etc.

If you have the duplexer that Motorola ships, look in the Quantar service manual behind the duplexer tab where you will find performance specifications. Compare all of your frequency requirements to the spacing capabilities listed in the table to determine if this is practical.

The ideal solution is a second Quantar for the second freq - or maybe an MTR for the analog freq and keep the Quantar for digital. Mixed mode conventional is just too confusing for users.

They do not have the typical MOTO in-cabinet duplexer that I have seen with the other Quantars I've installed. I believe it is an Andrew, 4 large cans in a big black cabinet. I have the model# laying around somewhere..... as to your suggestion about a seperate unit, that is exactly the suggestion I have made. They still have their old repeater (MSF5K) in the shack, collecting dust. We just so happen to have a VHF antenna not being used. I have told the guy that his best bet would be to lease our antenna, and either bypass the duplexer (seperate TX and RX antennas and set up 2 seperate channels in the Quantar) or better yet use both repeaters (Quantar digital, MSF analog), but so far he is whining about the monthly $ for another antenna. I'm trying to talk my boss into making them a deal on leasing our antenna since they're already paying full pop for their existing antenna. I REALLY didn't want to split the difference with the duplexer tuning.

Thanks all for the responses.

[Dan562]

Equinox,

Since you've only provided the two transmit frequencies 155.5875 MHz and 155.700 MHz, then you should add these frequencies together equaling 311.2875 MHz. Then you will divide by two equaling 155.643750 MHz, the Tx tuning center carrier frequency for your Duplexers and each RF Tx F1 & F2 Carrier Frequencies will be exactly +/- 56.25 KHz Offset from the new center of the tuned duplexer frequency. The Tx P.O. will degrade possibly .5~.7 dBm but that's only 10~15% of full RF Power Output.

You will be required to do the same calculations for the two Rx F1 & F2 Carrier Frequencies so the Quantar's receiver and Duplexers work correctly by Notching the Tx noise and Desense levels. As I recall sweeping Duplexers in /\/\ Engineering 25 years ago, the Bandwidth at -70 dBm was 250~300 KHz wide. You may experience an additional 1 dBm of Receiver Insertion Loss.

Nothing like a customer having enough knowledge to be dangerous trying to make a fool proof Mixed Mode "CAI" Quantar VHF Repeater do something it wasn't designed to do. This should have been a SP from Schaumburg's Quantar SP Bid & Quote Development Group.

Dan

[wavetar]

Dan is correct, my numbers were off...I was seeing 287KHz separation when it should have been 112.5KHz. This makes it better for you if you have to tune for in-between. It should work for you, I've set-up several UHF trunked systems by tuning for the center frequency, and it covers 4 or 5 repeater pairs with little issue.

Todd

[N3IVK]

"Or a TKR750..."

Try to keep with the topic, keep the off brand recommendations off the board.

[tvsjr]

"Or a TKR750..."

Try to keep with the topic, keep the off brand recommendations off the board.

Again, FOAD. Do you have some sort of problem with me?

Oh, and this would be ANOTHER two-month old post.

[wavetar]

Indeed N3IVK, dredging up a 2-month old post specifically to tell someone what to post/not post adds no value and can only be seen as antagonistic. Please do not do this. You can use the "report post" feature to bring a post to a moderator/administrator's attention. If we feel it requires further action, we will take it.

Todd

[Equinox]

Guess I should update since this popped up. I wound up talking them into leasing our antenna for the old analog MSF5K. Just retuned the new duplexer today for the new digital freqs on the Quantar. Hope to have them talking digitally asap. Thanks again for all the suggestions.