Does a TKR-850 need a pass/reject cavity filter?

[serial14]

I have a TKR850 repeater that has both a duplexer and a Motorola T1501AL pass/reject cavity filter in the RX path. It previously served as a business repeater in a metro area, so this made sense. It's now going to a remote mountain ranch where there is little RF in the environment and certainly nothing close by. Would it be good to keep the RX pass/reject cavity filter in? or would it do just fine given the low RF environment with out it? From a mechanical packaging standpoint, it would be advantageous to ditch the filter.

 

[prcguy]

If the repeater has a good band pass/band reject duplexer with 85dB+ rejection then it should be fine in a low RF environment without any additional filtering. If its a mobile flat pack duplexer it could go either way as I've seen those cause some desense even on a test bench. The extra cavity filter will add at least .5dB loss plus cable loss so leaving it out may improve receive slightly. That's assuming the cavity filter was in the rx path.

With a good duplexer you can consider using a low to moderate gain low noise preamp at the repeater receiver and then you would want an extra rx filter that will have as much rejection at +/- 5MHz as the preamp has gain. For example, if the preamp has 15dB gain then you want the additional filter to attenuate the transmit frequency at least 15dB at the receive frequency. I would also attenuate the output of the preamp to limit gain to about 8-10dB max.

 

[serial14]

It does have a small mobile flat pack duplexer. I need to unmount it to find part numbers from it still. Just about to do that actually.

As you also said.. It does have an ARA preamp in the RX path. Your explanation matches what I was thinking as well.

 

[serial14]

I just got the duplexer pulled off to inspect. Not many markings. A sticker for "Made in Canada" and a stamp that says "SRL 6". Overall, the build quality of the duplexer seems good. All the aluminum has been DA sanded and Alodine coated like you'd see in military and aviation type equipment.

 

[kayn1n32008]

I just got the duplexer pulled off to inspect. Not many markings. A sticker for "Made in Canada" and a stamp that says "SRL 6". Overall, the build quality of the duplexer seems good. All the aluminum has been DA sanded and Alodine coated like you'd see in military and aviation type equipment.

Sinclair duplexer. Should be 6 cavities. It will work okay if your in a low noise environment. Definitely keep the additional filtering in the receive path if you are going to use the preamp.

 

[prcguy]

It does have a small mobile flat pack duplexer. I need to unmount it to find part numbers from it still. Just about to do that actually.

As you also said.. It does have an ARA preamp in the RX path. Your explanation matches what I was thinking as well.

ARA might have been an ok preamp in the 1970s but its bottom of the barrel specs these days, very easy to overload those with its poor IP1/IP3 specs. However if its working on a test bench with no desense then it should be ok at a lot RF site.

 

[AK9R]

ARA might have been an ok preamp in the 1970s but its bottom of the barrel specs these days...

What do you like for UHF preamps on repeaters?

 

[prcguy]

What do you like for UHF preamps on repeaters?

I still have a few high level Angle Linear PHEMT preamps but Chip Angle retired and these are no longer available. Specs are:

Input & output return loss typically better than 16 dB

Noise Figure about .75dB

Gain 23dB

Output Intercept pt. (IP3) +39 dBm (450 to 500 MHz) & +38 dbm at 400 MHz

Output 1dB comp pt. >+25 dBm

Cost at lot in its day

Since any preamp I use on a repeater has quite a bit of brick wall filtering ahead of it I have been using MiniCircuits ZX60-P103LN with the following specs and I've used these on both VHF and UHF repeater systems padding the gain down to 8-10dB for a single repeater.

Noise figure .4dB

Gain 20-22dB

IP1 22dBm

IP3 39.7dBm

Cost $114

The MiniCircuits is very broad band and cheap compared to the old Angle Linear and even the ARA. One preamp model for both VHF and UHF is handy. BTW the typical ARA UHF preamp specs are:

NF .5dB

Gain 18dB

IP1 12dBm (dog doo doo)

IP3 not listed (probably too embarrassed or they don't know how to calculate)

Cost $160 (are you kidding me???)

 

[wd8chl]

There's no need for a preamp on a TKR-850. They have more than enough sensitivity on their own, and adding a preamp will most likely cause problems. You wind up with too much gain. Take the preamp out. You'll find far less issues with noise, intermod, and so on.

 

[fontanei]

- Well, how do you know its new home will be a low RF environment?
- Have you measured the background RF environment (BRFE) near the repeater's RX frequency?
- Same operating frequency (RX and TX)?
- Are you using the same antenna and transmission lines, from the old home, in its new home?

I say you go ahead and measure the insertion loss (S21) across the filter/reject cavity and decide from there if losing tenths of a dB will SIGNIFICANTLY degrade the receiver's probability of signal detection.

.....it appears that the signal path from normal users is basically line of sight to the receiver....if the transmission line's length is significantly less in its new setup I'd take out the preamp and leave the filter/reject cavity in-line just as a precautionary measure.

What acceptable SNR are you willing to live with at the receiver's input?

 

[serial14]

- Well, how do you know its new home will be a low RF environment?

Couple reasons. 1, I've operated radio systems at this ranch before. 2, FCC license search. 3, Terrain analysis. 4, Long duration scanner observations, recordings, & Spec An sweep performed several spots on the property.

- Have you measured the background RF environment (BRFE) near the repeater's RX frequency?

Yup

- Same operating frequency (RX and TX)?
- Are you using the same antenna and transmission lines, from the old home, in its new home?

Nope. All new stuff

I say you go ahead and measure the insertion loss (S21) across the filter/reject cavity and decide from there if losing tenths of a dB will SIGNIFICANTLY degrade the receiver's probability of signal detection.

.....it appears that the signal path from normal users is basically line of sight to the receiver....if the transmission line's length is significantly less in its new setup I'd take out the preamp and leave the filter/reject cavity in-line just as a precautionary measure.

Much shorter coax runs and significantly better quality. The original system used RG58( It was not under my ownership or control at the time ).

What acceptable SNR are you willing to live with at the receiver's input?

Yeah, this is a significant design variable for me that has flexibility. Its ok for some spots to have snow/crackle on the signal as long as the human voice can still be interpreted by trained/experienced operators. On the flip side... minimizing SWAP & complexity of the repeater setup gains me a lot in terms of reliability and the ability to even get it to where it needs to be.