Batboard

Our ham club is currently operating a Motorola Micor VHF repeater, with a RFS 497-1-1 6-cavity duplexer, and RFS 200 Super Stationmaster antenna. Output power from our PA to duplexer was recently measured at 120 watts.

Although the duplexer is rated at 350 watts, we recently read that the combination of a transistorized PA and a frequency separation of 0.6 MHz will almost always cause receiver desense when power output in excess of 100 watts is measure (due to broadband transmitter noise).

We do have a receiver issue of some sort that we haven't been able to pinpoint yet. The station can be heard about 15-20% further than 50 watt mobiles can talk back to it.

Might we experience an improvement if we drop the power down by 20-30 watts?

Comments?

I doubt that 20-30 watt drop in power will accomplish much unless there's something wrong with the transmitter.

I think you'd be better off investigating the actual performance of the repeater first with real test equipment. Start with the transmitter off and generate a signal through another antenna to give you 20dB quieting (or 12dB SINAD) at the receiver. Then turn the transmitter on and see if the received signal degrades at all. If so, then you are getting desense and you need to fix it. Lowering power may help a bit, but if you go too low, the PA may put out spurious emissions instead of wideband noise.

Make sure the duplexer is properly tuned, presents a 50 ohm impedance, and that you are getting the rated (or better) performance out of it. You want to use double-shielded coax between the repeater and the duplexer, such as RG214. Sometimes the cable lengths are critical; it depends on the equipment.

The 6-cavity duplexer should be able to handle the configuration you have with ease. The Micor receiver is certainly tight enough and the whole system might even work with just a 4-cavity duplexer.

If you're looking for balanced coverage with a 50w mobile, then you probably want to run a 50w repeater transmitter as well.

Bob M.

You may see an improvement, but rather than guessing at it I would head out to the site with a trusted service monitor or at least a good signal generator and make some receiver measurements. Check your receiver sensitivity both with the pa keyed and also unkeyed and make your adjustments as need be. I know its a crude way of doing it but it'll get the job done.
Doug
I see Bob beat me to the punch again

^^ What they said.

My RACES repeater is running 150 watts forward into the duplexer... standard 600KHz split, and no desense issues. I do have a dual-stage isolator on the transmit side, as well as a 4-pole crystal filter and receiver preamplifier on the receive side.

One of my fire repeaters is running 100 watts into the duplexer... 515KHz split (154.355/153.800), also with no desense issues. Same deal... duplexer is an EMR 6-can pass/notch with dual-stage isolator... receive side has a 4-pole crystal filter and receiver preamplifier (18dB).

I've heard that some pa's actually can become DIRTIER with a reduction in power. I'd say that if the pa is running at "design" and you have desense, then fix the problem, don't try and diddle the power level. There are lots of things that can cause desense, including cables, connectors, something amiss in the duplexer, poor shielding--loose, missing or improperly designed/installed, as well of course as something actually wrong with the pa. If it's a tube, could be the tube, power supply, or improper tuning.

I have found that there are one too many sites plagued with problems not related to the equipment inside the building, but rather the antenna, other antennas, or structures around it.

Check out the below article:
http://www.repeater-builder.com/antenna/cracking.html

This not only has 'crackling' as the symptom, but straight desense resulting in degraded RX also causes this and it may or may not be intermittent.

First I must ask, "Who in the world measured your Tx RF Power Output on the solid state Micor Power Amplifier producing 120 Watts!?

A Micor Base Station / Repeater C73RTB Intermittent Duty Solid State RF Power Amp is rated at 110 Watts.

A Micor Base Station / Repeater C73RCB Continous Duty Solid State RF Power Amp is rated at 105 Watts.

There's some thing not quite correct with the measurements that were performed at the Ham club's repeater site. Maybe, the RF Wattmeter wasn't a Bird 43 or the person making the measurements wasn't using a known good 50 OHM Dummy Load or the Wattmeter was inserted into the coaxial feedline prior to the Duplexers at an odd multiple of the wave length causing a high VSWR / RF Reflected Power. If it's true with the RF Power Output measurement, that's an excellent way to decrease the RF Transistor Life Span on the 4 final transistors which are No Longer Available from /\/\'s National Parts Department.

/\/\'s Engineering specification regarding a reduced RF Power Output from their Micor solid state transmitters. The RF power amplifiers are allowed up to a -3 dBm or 50% reduction of RF P.O. before Spurious emissions are possible to appear across the RF spectrum. Stating this and using a 105 W RF Power Amplifier, then the P.O. can be set any where in-between 52.5 W to 105 W and be free of Spurious emissions. I would recommend to set the RF Power Output at 75 Watts for longer life on the transmitter and the small change in output power will hardly be noticed in the over all coverage change by the mobiles or handheld portables using the repeater.

What you first need to do is take a known good Service Monitor out to the repeater site and make the critical receiver sensitivity measurements. The VHF Micor station's receiver is specified at .5 microvolts at 20 dB Quieting or .35 microvolts at 12 dB SINAD without a Rcvr Pre-amp. The .5 uV @ 20 dB Quieting (Q) could normally be tuned down to .4 uV @ 20 dBQ. The receiver's Discriminator should Idle at +/- 2 microamps (uA) Maximum without a signal present, they normally will Idle at 0 or +/- 1 on Meter # 4 on the Metering Test Panel.

Always use Double Shield Coaxial Cable RG-214 between the transmitter & receiver antenna connections and the Duplexers. Always utlize the proper shields on the Exciter, Power Amplifier and Receiver and the larger shields that cover the Transmitter and Receiver to reduce chances for interference.

There are many things that can affect repeater receiver coverage such as: Site Location / Antenna Height Above Ground Level or Height Above Average Terrain, Path Losses such as Hills / Terrian, Trees / Foliage, Coaxial Feedline / Heliax and Buildings. Even co-users on a particular tower site can affect your repeater system coverage. You have to take in to consideration what the mobile antennas are being used and placement on the vehicles, which will affect the RF signals transmitted to your repeater site. What are you or your membership expecting for the Ham club's radio coverage ... the world wrapped around with fence?

Go back and do some homework as to the repeater antenna height, what type of coaxial feedline and total lenght between the duplexers and antenna, the receiver's sensitivity, if the tower site is colocated with commercial users and the true RF P.O.

Dan

After you have considered the other posts, especially the previous poster regarding your station parameters...Consider...

TITO = Talk In Talk Out balance. You only require 50W base station TX power to be heard by a 50W mobile in the same quality that the base station hears the mobile.

VHF in urban areas is NOISE limited. More antenna gain will get your TX signal out farther (OK for paging, but not for two way), but will not likely result in a useful improvement in EFFECTIVE receiver sensitivity. Desense of 6 to 10 dB in urban areas for 12dB SINAD comparison between receiver with and without antenna... considered normal! This is why my company uses only 5 dBd gain VHF dipole antennas.

The mobile might not have these desense issues on the ground and thus would be more sensitive to hearing the distant repeater TX.

RFDude