Pre-amp

[kayn1n32008]

Working on a problem. I have a repeater, combined with 2 others, to a single antenna. Total system losses for the repeater i'm interested in is 4.7dB.

Would it be worth adding a pass can(1.5-3dB of additional loss) between the Duplexer and receiver, with a 9dB gain preamp to make up for the system losses? The total system losses with the additional pass can would be between 6.2 and 7.7dB. I only want to make up for the Duplexer and multicoupler/combiner losses. I do not want to make it hear farther than it can talk.

The repeater of interest is up at tx171/rx172 actual separation is 0.735MHz. The other two repeaters are 169/169(0.435MHz)and 157/162(5.340MHz)

 

[prcguy]

Usually a receiver combiner system has a preamp to overcome splitter losses, are you sure there is not one already? You cannot overcome duplexer loss, its ahead of any preamp or receiver and will be part of what sets the noise figure of the entire receive system.

But you can improve system noise figure by overcoming splitter loss with a preamp ahead of the splitter if there is adequate preselection before the preamp and adequate post filtering on the transmitter(s). If the system is working properly now then you would need a preselector that has at least as much attenuation at the transmitter frequencies as the preamp has gain, and the preamp would need an IP1 or IP3 rating high enough to not produce any measurable IMD in your receive frequency range from signals within the preselector passband and also transmitter leakage and broad band noise getting into the receive system.

You would also want the least amount of insertion loss as possible on the preselector because that adds directly to the system noise figure.

Looking at your frequencies again just now (I missed some important details!) its not possible to add a preselector because you have 3 receive frequencies sandwiched between all the transmit frequencies and you would have to place a preselector and preamp in front of all receivers to overcome splitter loss. That would require a very custom and expensive preselector that would pass just the 3 separate receive frequencies while giving adequate rejection from a transmitter .435MHz away from one of the receive freqs and .735Mhz from another.

If you place a preselector and preamp at the output of the splitter just for your one receiver you will have more signal into your specific receiver at the expense of raising the system noise figure and the end result will probably be going backwards. Combining all the transmitters on one antenna is easy but for the best performance I think each receiver should have a dedicated antenna with appropriate preselection.
prcguy

 

[kayn1n32008]

I have not laid eyes on the shack yet. I am only interested in one of the three repeaters.

My belief is that all three repeaters likely all have their own duplexers, and they are all muti-coupled to the same antenna, rather than having a transmit combiner and receive multi-coupler.

Sadly there is no chance of putting each repeater on their own antenna. The repeater I'm interested in, 171.xxx/172.xxx transmits at 13.3dBw, as does the one in on the 169.xxx pair. The other transmits at 11.1dBw. The power levels are not all that high, which is why me thinks it is a multi-couple system, rather than a transmit combiner/receive multi-coupler.

This is a bit of a side project. There is a push to add two more repeaters, all in-band linked with the 171.xxx172.xxx existing repeater would be the 'hub', to augment and improve coverage.

This repeater just is not in the ideal spot to cover the area of operation, and I do not believe there is a single site that would provide the coverage wanted. Moving it is not really an option, even though there is another site that would fit the bill. But still not provide the coverage needed for a single site.

I am already going to recommend swapping the 1/4 wave SRL 210-C4 for a 1/2 wave spaced SRL 210-C4. I am also recommending shifting the element azimuth from 0deg true to 45deg true to improve coverage directly behind the tower, by putting the null to the 'short' side of the area of operation. A new repeater will fill the null, created by being side mounted, below the top of the tower. Raising the antenna is not an option.

Another recommendation is to add one of the two new repeaters in the blind spot of the existing one. .

 

[prcguy]

I've seen a lot of repeater sites but can't say I've seen multiple transmitters and receivers sharing the same antenna. For it to work on VHF with .435 and .735MHZ splits there has to be some very good duplexers and if conventional pass/notch they would be at least 6 cans each and all duplexers would connect to a multi output or star connector right to the feedline. Can you check on that?

If that's how its put together then you would only have duplexer loss and no splitters, etc, so minimal loss between the antenna and receivers. In that case adding a can or two to the receive output of the duplexer then to a good preamp might be beneficial but don't expect any night and day difference.
prcguy

 

[prcguy]

Just to ramble a bit more, there might be broad band noise from the other transmitters on the same antenna when they key up that degrades "your" receiver and you might need a spectrum analyzer to verify that.

Many of my repeater sites in southern California are world famous for their radio density and high RF noise floor. Adding a low noise preamp at those sites doesn't do much except shove more signal into the receiver and make it more susceptible to overload. The noise floor is so high that it sets the weak signal receive level at a much higher point than the repeaters system noise figure.

At other sites in RF quiet areas adding a preamp with appropriate preselector ahead of it does improve weak signal reception to hand helds and distant stations. Before buying and installing preselectors and preamps you should at least measure what your receiver is seeing with a spectrum analyzer to see if its worth doing.
prcguy

 

[kayn1n32008]

My guess, and at this point it is a guess, is that the repeaters are Using Q2330E 6 cavity Resloc's with a multi coupler string consisting of at least 3 pass and a notch per section(there would be three) of string.

Thankfully there is no other VHF transmitters with in a mile of this site, and everything else on this site that emits RF is from 700MHz up to 1900MHz. Eventually 2100MHz transmitters will be added. I think that can explain why mor antennas and moving the existing one is not an option.

As soon as I can gain access to the shack I shall provide an update.

Thank you for your input.

I have my doubts any recommendations will be followed for this site, but want to give the owner all the options and let them decide. I fully agree that a pre-amp is a last resort and may not be an option here.