Repeater with Telewave Duplexer question

[meyer]

Hi to everyone.

I have a rather unusual question.

I have a repeater on a commercial tower. It is currently running on a 9Db Station Master at 143 feet. There is an recently abandoned DB420 that I have access to. It is located at 150 feet. I have tested the repeater on the DB420 and the RX is nearly twice as good but the TX is far weaker. My SWR is about 1.6   I suspect he DB420 is a little off on frequency and may be the cause for the poor TX performance.

Here is my question

I have a 4 can Telewave TPRD 4554 duplexer. Can I simply plug the DB420 on the 2 RX side cans and the Station Master on the 2 TX cans? This would meant I remove the combining jumper in between the cans where it goes into 1 antenna. 

Meaning I would have a seperate TX and RX antenna but keep the duplexer to avoid desense? 

Any help would be appreciated!

Thanks 

[marcspaz]

The answer is yes and no.

 

Lots of people use two antennas and split duplexers. You can absolutely separate any/all of the cavities and arrange them in any order and use multiple antennas. The cavities are identical and all capable of being use separately or stacked in any pairing number.  However, the cavities are all tuned to work together. There is an extreme likelihood that you will have to re-tune the duplexer. In my experience and opinion, the chances of splitting the cavities apart and not negatively impacting filtering without re-tuning is almost zero.

 

So, if you can tune it or have someone who can tune it for you, go for it.

[gortex2]

Simpler solution is to move the new antenna to the duplexer and put a dummy load on your TX port. Then run your TX antenna directly to the repeater. You dont need the filter on the TX side and you also wont affect the RX foltering by just not using the TX side of the dulexer. Putting the dummy load on it will eliminate any interfearance. All of my GMRS repeaters are on multi couplers that just have bandpass filters and TX line direct to antenna. 

[marcspaz]

4 hours ago, gortex2 said:

Simpler solution is to move the new antenna to the duplexer and put a dummy load on your TX port. Then run your TX antenna directly to the repeater. You dont need the filter on the TX side and you also wont affect the RX foltering by just not using the TX side of the dulexer. Putting the dummy load on it will eliminate any interfearance. All of my GMRS repeaters are on multi couplers that just have bandpass filters and TX line direct to antenna. 

 

The Telewave is a Pass/Reject filter. The high-side set of cavities are considered one set of filters and the low-side is a second set of filters. Getting rid of the transmit pass filter and receive reject filter on the transmitter side will 100% impact receive sensitivity and cause other issues.

Even though the repeater does not (intentionally) transmit on the receive frequency, the transmitter's output can still interfere with the receiver due to things like transmitter leakage (aka Broadband Noise), failure to isolate the transmit and receive paths leading to small amounts of transmitter power leaking into the receiver (even on separate, but close proximity antennas), and overloading the receiver’s front end with all the transmitted noise (spurs, harmonics, etc.)

The receiver’s front end is designed to detect very weak signals. Any leakage from the transmitter (even if it’s not exactly on the receive frequency) can overload the receiver’s front end, causing distortion and reducing the ability to detect the intended signal.  The notch filter and pass filter acts as a barrier, greatly attenuating any transmit signal that might overlap with the receiver’s frequency range.

To try to overcome the lack of filtering on the transmit side, you can stack all 4 cavities in series on the receive side, but they will all need to be re-tuned and the end result would still not be anywhere near as good as if you had the transmit properly filtered. 

[SteveShannon]

3 minutes ago, marcspaz said:

 

The Telewave is a Pass/Reject filter. The high-side set of cavities are considered one set of filters and the low-side is a second set of filters. Getting rid of the transmit pass filter and receive reject filter on the transmitter side will 100% impact receive sensitivity and cause other issues.

Even though the repeater does not (intentionally) transmit on the receive frequency, the transmitter's output can still interfere with the receiver due to things like transmitter leakage (aka Broadband Noise), failure to isolate the transmit and receive paths leading to small amounts of transmitter power leaking into the receiver (even on separate, but close proximity antennas), and overloading the receiver’s front end with all the transmitted noise (spurs, harmonics, etc.)

The receiver’s front end is designed to detect very weak signals. Any leakage from the transmitter (even if it’s not exactly on the receive frequency) can overload the receiver’s front end, causing distortion and reducing the ability to detect the intended signal.  The notch filter and pass filter acts as a barrier, greatly attenuating any transmit signal that might overlap with the receiver’s frequency range.

To try to overcome the lack of filtering on the transmit side, you can stack all 4 cavities in series on the receive side, but they will all need to be re-tuned and the end result would still not be anywhere near as good as if you had the transmit properly filtered. 

This is such a clear explanation that I actually mailed it to myself for future reference.  Nicely explained, Marc!

[gortex2]

I have the exact duplexer on 2 of my repeater sites Yes each can is a PASS/REJECT. I am doing exactly what I said at one site with no decense or issues. RX antenna into duplexer. RX duplexer port to RX on Quantar. TX port on duplexer with dummy load. TX Antenna to TX port on Quantar. My seocnd site has 2 of the cavities on the RX side only (PASS 467, reject 462) with no issues. It a cavity. We use those same exact units in multicouplers all the time (in Public Safety). If you put the VNA into the antenna port on the duplexer you should see no change regardless of a transmitter or dummy load. If you do the duplexer isn't tuned correctly. 

 

[marcspaz]

I have a Telewave TPRD, too.  And a few other brands.  What I am saying is that there are caveats to be aware of and action needed for optimization.  There is a reason that the industry standard is to filter transmit and receive. 

The mere presence of one cavity impacts the other cavities.  That is why the tuning instructions say to tune each cavity individually first, and then re-tune them collectively with all cavities connected.  Needing to re-tune when changing the cavity configuration doesn't imply that it wasn't tuned correctly in the first place.  That is the nature and expected response of stacking filters.  

I do agree that removing the transmitting side of the radio from the duplexer and adding a dummy load should not impact tuning, since that is how we tune them to begin with.

Also, I'm sure you're setups are working fine.  I would be willing to guess there are other factors at play, such as there is desense in your system, but the system performs well enough that the degree of impact isn't as critical.  Or the transmit and receive antennas are significantly physically disperse. Somewhere between 400 and 600 feet of horizontal separation to accomplish an additional -67dB to -70dB of attenuation, or 20 to 30 feet of vertical separation to yield about -42dB to -44dB of attenuation, which isn't horrible if you have a filter on the receive side and you are running low power (<50w).

[meyer]

You are 100 % correct. I have went to the site today before I seen you post and put a dummy load on the TX side of the Telewave and plugged the TX antenna into the PA of the repeater. It was unable to receive and just kept dropping out when someone would key up with no audio. Obvious desense. Oh well, it probably not worth the trouble to get it retuned on site to adapt as my access to the DB420 could go away at any time. 

Thanks you all for your help with this. What a learning experience!

[marcspaz]

@meyer sorry to hear it didn't work out. Im not sure where you are, but im in the DC .metro area. If you're close enough and want to pursue it, I would be happy to help.

[meyer]

Cincinnati 

Its a very heavy RF site with 5 other towers around it. Mostly microwave and appears 800Mghz (I think) in the room that we have. I'm sure this had some factor in the failure. I am in the works now of a second site. I have to attend another City meeting for final approval and it could all fall apart but it would be great to consult with you guys regarding the antenna choice and setup if that's okay?

Thanks again Gortex2 Marcspaz for the information WSCP709 used to be WQXC777 Cincinnati 7250

[marcspaz]

2 hours ago, meyer said:

it would be great to consult with you guys regarding the antenna choice and setup if that's okay?

 

Yeah, man. We ha e a good group here. I'm happy to help where I can. I'm sure others will continue to help, too.

[WRTH852]

Just some food for thought.... You might want to put an antenna that is closer to your transmitting freq up on the tower.  An SWR of 1.6:1 thru 150 ft of line (you don't say what kind of cable or how old/good of shape its in) is a bit concerning.   Depending on the loss of the cable (you can look it up), you will loose a part of your transmitted power going up the tower, and that lower power at the top will see the mismatch at your antenna (SWR), and a part of that reduced Tx power  will be reflected back due to the mismatched impendences .   That reflected power will also see the same amount of loss going back down the cable to the repeater.  This causes the SWR to look  better than it really is up there at the antenna due to loss's in the 300 ft round trip.   I would suggest getting an antenna that is tuned for your transmit frequency and put it up there if you can get someone able to climb for you,  so you can make best use of the power that does get up there.  Also while they are doing that, a check to see that the cable is not damaged might prove helpful. A dummy load at the top and a SWR reading or TDR test while you have a climber could be informative.   Depending on where the tower is located falling ice ,  hail, or lightning strikes on the tower could have done something nasty to your cable.     Just some ideas for your consideration... I hope its helpful to you..