New Repeater Configuration Proposal(s)

[Luke Jenkins]

After much reading and coming across a couple of nice deals on repeater gear*, I think we're getting close to being able to reconfigure the repeater and get 145.25 back up on the tower and better than ever.

*KE7BAP found a KSL.com listing for 12 band pass VHF cavities for $500. This was too good of a deal to pass up; so he, KE7VVT and I split the purchase three ways. If we end up with extras, we'll sell them on and recoup some of those costs. I've also picked up a few more Motorola Spectra and Radius mobile radios that have better RF front ends than our current Yaesu mobiles. Thanks to Dr. Sohl's 486, we're now able to reprogram these.

After reading this passage:

Many sites use shared "community receive" antennas that feed a preamp/multicoupler panel - at one 5,300 foot mountaintop site I visit semi-regularly there is a 120 foot tower with heavy crossmembers every 20 feet. The top level is receive only - there are dedicated antennas for VHF, 220 MHz, UHF, 800 MHz, 900 MHz and 1200 MHz with hot spare antennas already in place for VHF, UHF, 800 MHz and 900 MHz. The UHF antenna and it's spare is a broadband (403-512 MHz) Sinclair with an AngleLinear preamp mounted at the antenna base. It feeds a run of inch-and-five-eights Heliax that ends up connected to an AngleLinear UHF multicoupler panel. Each port of that multicoupler panel connects to the receiver in a UHF repeater (over 35 of them). Likewise there is a single broadband (132-174 MHz) high band Sinclair antenna (with another tower top preamp) feeding another run of inch-and-five-eights Heliax to a VHF AngleLinear multicoupler panel that feeds all of the high band receivers. In both cases the multicoupler panel has a power injector assembly that feeds DC up the the feedline to power the tower-top preamp.

 

of http://www.repeater-builder.com/antenna/ant-sys-sys-engr.html and speaking with KZ7O who is on the OARC repeater team yesterday about running multiple transmitters on our tower, I had an idea.

I think we should run the numbers on the following configuration:

It is a modified dedicated receive where we transmit 145.25MHz on the top, multiple receive, antenna. The second transmitter, APRS on 144.39MHz, will use a second antenna located about half way down the tower. This will get us maximum height on our receive antenna, while keeping the two transmitters on separate antennas. 15' (+/- to try to get the antennas into a voltage null of each other) should get us ~42dB of isolation per http://www.repeater-builder.com/antenna/separation.html . The exact configuration and number of filters we need to use still needs to be worked out. I think the weakest point is that we don't have a pair of notch filters notching out 144.650MHz spurious emissions from the 144.39MHz transmitter.

This design should get us maximum receiver range and sensitivity on both 144.65MHz for the voice repeater and 144.39MHz for APRS, without sacrificing too much in the way of 145.25MHz or 144.39MHz transmitter coverage of the area.

I'd love to get some feedback on this idea. If anyone has any other configurations that get us both the voice repeater and APRS up on our ~30' tower at the same time, please diagram them up and share them. Assume as many radios and antennas as are needed, 6 notch filters, 12 band pass cavities, and any other reasonable expenses are available.

-Luke
KD7FDH

[Jonathan Karras]

Will the notch filters have unexpected side affects where they tee of the coax instead of pass the signal through them like the band pass filters? Meaning will the .25 notch filter on the lower portion of the diagram really end up filtering everywhere?

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[Luke Jenkins]

There shouldn't be any side effects of having a tee and multiple receivers behind a set of cavities.

-Luke

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[John Metcalf]

Luke,

I think this is great! I'm not convinced that we need all of the cavities though. Each cavity will:

1. Provide 19+\- 4 dB at the notch or pass

2. -0.1 to -2 dB across the entire spectrum do to insertion loss even at the tuned frequency

3. Another 5 +\-2 dB of insertion loss do to the frequency response. The notch filters continue to attenuate at higher frequencies instead of returning to zero. Not sure if this is an issue with bandpass though but we did notice that the frequency response was more ideal at lower frequencies. Just something to keep in mind.

I agree that we can't have the aprs tx and repeater rx on the same antenna, but what about aprs tx and repeater tx on the same antenna, and rx aprs and rx repeater on the other? This would give us the greatest sensitivity for the receiving radios. Would there be issues with two transmitters txing at the same time? 

Aside from this config having an overly conservative approach in terms of isolation, I think it is well thought out and will definitely get the system on the air. If we have time to get the isolation analysis done later we can think about removing certain cavities to improve rx and tx performance. Aside, if the isolation analysis shows we can't remove cavities I think that you have already chosen the optimal configuration!

Once this is up I would want to see some rx s- meter and signal reports (both ends) for mobile and handheld radios at different power levels at different locations in the city. From that we can determine if our system has 'all ears' or 'all bite', as the repeater builder put it, and reconfigure again to get the right balance for rx sensitivity and tx range.

Let me know what I can do to help get your proposed config going.

John M - ke7vvt

Sent from my iPhone

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[Luke Jenkins]

I agree that we might not need all of the cavities I have on the diagram. I also think that we might end up needing additional cavities, especially on the APRS TX side of things to keep it out of the 144.650 RX radio if the vertical separation of the two antennas isn't enough. We won't know until you finish crunching numbers like you wanted to, or we get the cavities all tuned and run some tests.

The idea of having a dedicated RX antenna up top with a per-amplifier and one TX antenna lower down would be ideal. However, if you put more than one transmitter on a single antenna you end up with intermodulation products from the reflected energy going into the power amplifiers of the different transmitters, creating all sorts of issues. If we hooked up our 145.25MHZ and 144.39MHZ transmitters to the same antenna, we'd have intermod energy going out on at 289.64MHz (a government exclusive band) and other frequencies.

The solution to this, and how the big guys do it, is with RF circulators/isolators. They are cool guys where energy that goes in Port A can only go out Port B, Port B out Port C, and Port C out Port A. So you put your transmitter on A, your antenna feed line (well through a low pass filter and possibly a bandpass cavity first) on Port B, and then a dummy load on Port C. Transmitted power goes out Port B, and any reflected energy (from the transmitter in question or any other in the system) gets circulated out to the dummy load on Port C. They are rather spendy, but worth considering. http://www.repeater-builder.com/antenna/ant-sys-index.html#iso-cir has good info.

-Luke

On Monday, September 15, 2014 6:56:12 PM UTC-6, John Metcalf wrote:

Luke,

I think this is great! I'm not convinced that we need all of the cavities though. Each cavity will:

1. Provide 19+\- 4 dB at the notch or pass

2. -0.1 to -2 dB across the entire spectrum do to insertion loss even at the tuned frequency

3. Another 5 +\-2 dB of insertion loss do to the frequency response. The notch filters continue to attenuate at higher frequencies instead of returning to zero. Not sure if this is an issue with bandpass though but we did notice that the frequency response was more ideal at lower frequencies. Just something to keep in mind.

I agree that we can't have the aprs tx and repeater rx on the same antenna, but what about aprs tx and repeater tx on the same antenna, and rx aprs and rx repeater on the other? This would give us the greatest sensitivity for the receiving radios. Would there be issues with two transmitters txing at the same time? 

Aside from this config having an overly conservative approach in terms of isolation, I think it is well thought out and will definitely get the system on the air. If we have time to get the isolation analysis done later we can think about removing certain cavities to improve rx and tx performance. Aside, if the isolation analysis shows we can't remove cavities I think that you have already chosen the optimal configuration!

Once this is up I would want to see some rx s- meter and signal reports (both ends) for mobile and handheld radios at different power levels at different locations in the city. From that we can determine if our system has 'all ears' or 'all bite', as the repeater builder put it, and reconfigure again to get the right balance for rx sensitivity and tx range.

Let me know what I can do to help get your proposed config going.

John M - ke7vvt

Sent from my iPhone

On Sep 14, 2014, at 18:07, Luke Jenkins <l.@gmail.com> wrote:

After much reading and coming across a couple of nice deals on repeater gear*, I think we're getting close to being able to reconfigure the repeater and get 145.25 back up on the tower and better than ever.

*KE7BAP found a KSL.com listing for 12 band pass VHF cavities for $500. This was too good of a deal to pass up; so he, KE7VVT and I split the purchase three ways. If we end up with extras, we'll sell them on and recoup some of those costs. I've also picked up a few more Motorola Spectra and Radius mobile radios that have better RF front ends than our current Yaesu mobiles. Thanks to Dr. Sohl's 486, we're now able to reprogram these.

After reading this passage:

Many sites use shared "community receive" antennas that feed a preamp/multicoupler panel - at one 5,300 foot mountaintop site I visit semi-regularly there is a 120 foot tower with heavy crossmembers every 20 feet. The top level is receive only - there are dedicated antennas for VHF, 220 MHz, UHF, 800 MHz, 900 MHz and 1200 MHz with hot spare antennas already in place for VHF, UHF, 800 MHz and 900 MHz. The UHF antenna and it's spare is a broadband (403-512 MHz) Sinclair with an AngleLinear preamp mounted at the antenna base. It feeds a run of inch-and-five-eights Heliax that ends up connected to an AngleLinear UHF multicoupler panel. Each port of that multicoupler panel connects to the receiver in a UHF repeater (over 35 of them). Likewise there is a single broadband (132-174 MHz) high band Sinclair antenna (with another tower top preamp) feeding another run of inch-and-five-eights Heliax to a VHF AngleLinear multicoupler panel that feeds all of the high band receivers. In both cases the multicoupler panel has a power injector assembly that feeds DC up the the feedline to power the tower-top preamp.

 

of http://www.repeater-builder.com/antenna/ant-sys-sys-engr.html and speaking with KZ7O who is on the OARC repeater team yesterday about running multiple transmitters on our tower, I had an idea.

I think we should run the numbers on the following configuration:

It is a modified dedicated receive where we transmit 145.25MHz on the top, multiple receive, antenna. The second transmitter, APRS on 144.39MHz, will use a second antenna located about half way down the tower. This will get us maximum height on our receive antenna, while keeping the two transmitters on separate antennas. 15' (+/- to try to get the antennas into a voltage null of each other) should get us ~42dB of isolation per http://www.repeater-builder.com/antenna/separation.html . The exact configuration and number of filters we need to use still needs to be worked out. I think the weakest point is that we don't have a pair of notch filters notching out 144.650MHz spurious emissions from the 144.39MHz transmitter.

This design should get us maximum receiver range and sensitivity on both 144.65MHz for the voice repeater and 144.39MHz for APRS, without sacrificing too much in the way of 145.25MHz or 144.39MHz transmitter coverage of the area.

I'd love to get some feedback on this idea. If anyone has any other configurations that get us both the voice repeater and APRS up on our ~30' tower at the same time, please diagram them up and share them. Assume as many radios and antennas as are needed, 6 notch filters, 12 band pass cavities, and any other reasonable expenses are available.

-Luke
KD7FDH

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