Noisy repeater output

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It could normal for the transmiter. How many notch cavities are you using for a duplexer >

Argh. You're overdriving the spectrum analyzer. Most of the junk looks like it's coming from intermod mixing between the harmonics produced in the now non-linear spectrum analyzer input states.

The SA shows that your input signal is 4.52dBm (about 3mw) into an internal 10dB attenuator. What model HP spectrum analyzer are you using and will it tolerate a +5dBm input signal level?

Put some external -10dB attenuators in line and see if the junk (noise) decreases. If it decreases more than -10dB, then it's being generated in the SA by your overload. If it drops exactly -10DB, then it's real.

Duplexer tuning and cabling are somewhat of an art. For example, the coax cables between the duplexer and the receiver input, and between the duplexer and the transmitter output MUST be an odd multiple of 1/2 wavelength electrical long, as measured from the pickup loop in the cavities, to the receiver front end device or transmitter RF output transistor. Use the wrong length, or wrong connectors, and you'll never get the rated isolation.

I agree with Ralph Mowery. Go to: and ask the same question, but also include what radio and duplexer you are using. Also what test equipment you have available. Most important, exactly what problem you are trying to solve that required troubleshooting.

There a many articles and videos on the web on "duplexer tuning". I would recommend one, but need to know the details of your repeater to find something appropriate.

Good luck.

Il 14/06/2019 05:09, Jeff Liebermann ha scritto:

I don't think so. The repeater TX has an output of 10W. Prior to enter in the spectrum analyzer, the signal is reduced by a 20 dB in line attenuator.

The SA input tolerate 1W (30 dBm) The model is 8590B

We have tryed with Procom and Motorola duplexers, both with 3 adjustable cavities for RX path and another 3 adjustable cavities for TX path. The same duplexers work perfectly with a repeater built with two separate Simoco SRM9000, without any desensization of RX unit when the TX is operated. Thus, I think that the PA of Vertex VRX7000 has some issues, but I never seen noises of that type. I wolud expect some harmonics. The internal supply can do this noise?

Ok, thanks, I will try also the repeater-builder forum, but if I can obtain some useful tips without register me I will much appreciate. Wednesday I will resume the repeater investigation.

Il 13/06/2019 22:04, Ralph Mowery ha scritto:

Thanks, please follow the answer to Jeff Liebermann

Please do the above test with a 10dB attenuator. If the spurious noise drops 10dB, then you have a real problem. If the noise drops more then 10dB, the noise is being generated inside the spectrum analyzer.

The spectrum analyzer picture shows that you are operating on 162.4 MHz. In the USA, that's one of the weather stations broadcast frequencies. I have no idea what 162.4MHz might be used for in Italy.

You haven't supplied any additional information on the equipment you're using, duplex split frequency, and available test equipment. The HP spectrum analyzer will survive the obvious overload, but I'm not sure if it will produce usable measurements (without spurious noise). As I mentioned, try reducing the input level with an additional attenuator and see if the spurs disappear.

I have used a HP 8924C for a SA and sometimes it is difficult to tell if it is overloaded at first even though it has the high power option and will handle 100 watts.

Sometimes to see the spurious RF you need to notch out the fundimental.

I've had the same problem with every spectrum analyzer I've used. When brand new, the SA is probably quite linear throughout its specified dynamic range. However, after getting overloaded with too much RF causing components to fry, the dynamic range can be reduced somewhat. It's difficult to tell if something is broken, but the 10dB attenuator test I previously mentioned is a clear indication that either something is wrong or burnt out.

Notice that the marker shows a signal level of 4.52dB (upper right of screen), but shows about +10dBm on the graticule. Kinda looks like the detector and/or the A/D converter that is measuring the marker level has gone rather non-linear.

That works very well on UHF frequencies which have a 5MHz split between transmit and receive. Unless you have a rather large cavity handy, it works badly on VHF frequencies, where the split can be as little as 0.6MHz. For the present situation, the SA shows a span of almost 300MHz for a 161.4MHz center frequency. For all I know, the spurious noise could be switching noise, AM/FM/SW/TV broadcasting, other services, etc along with their intermod mixes. With a sweep from 0 to 300MHz, that's about what I would expect with a cliplead crammed into the antenna connector.