low pass filter - rptr location

k4iii · Post by **k4iii** » Mon Feb 07, 2011 12:28 pm

I'm just wondering, has anyone ever put a low-pass filter (Say a Telewave, Celwave or Decibel Products ~200w) tunable unit between the duplexer and antenna? I know a low pass are used after a circulator to reduce harmonics on the transmitter port, and some repeaters even use high-pass filters placed on the receiver port between the duplexer and rx/tx side respectfully...

My "expertise", or "lack there of it" believes that putting this low-pass filter on the antenna port of the duplexer "should" knock out interference from 800 coming both in and out for both sides, right? Or will it cause desense to the repeater? Maybe it won't duplex very well?... Well, after all isn't one half of a diplexer unit low-pass and the other high-pass?

I guess I can always TRY it with my 2600 monitor... but simply wondering whether anyone has ever placed one of these tunable filters on the antenna port of a duplexer and with any success... Thanks.

Bill_G · Post by **Bill_G** » Mon Feb 07, 2011 1:05 pm

Are you operating 800M and need to reject another system physically close by that is interfering with your receiver through your duplexer? If so, who was there first - you or them? The one with the newest license gets to fix the problem.

Astro Spectra · Post by **Astro Spectra** » Mon Feb 07, 2011 2:19 pm

A suitable LPF is useful if you have a circulator on your TX output as you can get low level harmonics generated by the magnetic device.

Remember also that duplexer cavities work just as well on their odd harmonics! A duplexer can be high/loww pass but are more usually an asymmetric band pass or band pass with notches depending on the design.

The key questions are: what band is your repeater operating on, are you experiencing interference in your RX, and is there an issue caused by your TX?

Jim202 · Post by **Jim202** » Mon Feb 07, 2011 6:03 pm

k4iii wrote:I'm just wondering, has anyone ever put a low-pass filter (Say a Telewave, Celwave or Decibel Products ~200w) tunable unit between the duplexer and antenna? I know a low pass are used after a circulator to reduce harmonics on the transmitter port, and some repeaters even use high-pass filters placed on the receiver port between the duplexer and rx/tx side respectfully...

My "expertise", or "lack there of it" believes that putting this low-pass filter on the antenna port of the duplexer "should" knock out interference from 800 coming both in and out for both sides, right? Or will it cause desense to the repeater? Maybe it won't duplex very well?... Well, after all isn't one half of a diplexer unit low-pass and the other high-pass?

I guess I can always TRY it with my 2600 monitor... but simply wondering whether anyone has ever placed one of these tunable filters on the antenna port of a duplexer and with any success... Thanks.

Good engineering practice dictates that a filter be used after a circulator. In fact many site rental leases state that you must use a filter on your equipment to reduce the possibility that you will cause interference problem to another radio user already on the site. There are a number of other ways to reduce problems, but you have to be careful how you use them.

Jim

k4iii · Post by **k4iii** » Tue Feb 08, 2011 8:01 am

It is a single UHF system located at a high 800 rf site. (no circulator needed) I was simply wondering whether a uhf lp filter would work when placed between the UHF repeater duplexer output and antenna polyphaser, rather than purchasing one for each tx or rx side separately... I know how they are usually used on circulators, intermod panels... etc. I'm simply trying to use them for another application... to remove some 800 hash on multiple 800 frequencies from coming into the system, before reaching the duplexer. (The DB Products unit has .1db loss on rx freq and .2db loss on tx freq and can handle 200w)

The only issue (if that's what you want to call it) is the uhf has a higher noise floor due to the 800s. I'm simply trying to notch out some of the 800 hash before the duplexer, (from reaching my rx, while cleaning my tx) without adding too much attenuation inline.

I haven't tested it to see if it would cause desense or intermod where I am thinking of placing it. Has anybody seen one of these devices used for this purpose or in this location or know about how it would with a full-duplex signal passing through it (rx and tx simultaneously)? Thoughts? Experience?

Bill_G · Post by **Bill_G** » Tue Feb 08, 2011 8:31 am

What kind of duplexer are you using? If it is a pass/reject, then it should cut out 800M traffic as good as any lp filter you add. If it's a low cost reject duplexer (typically sold with R1225 and similar on site repeaters), then you should consider upgrading to a better duplexer because the 800M system will not be your only problem.

k4iii · Post by **k4iii** » Mon Feb 14, 2011 12:04 pm

The duplexer is pass/reject 1504 type. (Yes it does have the notch adjustments) However, duplexers for a band usually notch within the band well. If there is anything nearby outside it sometimes it helps, but when antennas are right next to each other, Interference outside the duplexer band can still hammer your repeater's front end. That is why on community sites, the owners often require isolation panels to keep signals from mixing, even when a duplexer or filter cavities are used...

I will just have to try putting the box inline with duplex signals going through the filter to see whether desense is caused by running full-duplex signals going through it between the polyphaser and duplexer antenna port... Thanks again everyone...

AEC · Post by **AEC** » Thu May 12, 2011 11:56 am

I would run calculations of ALL channels on that site, from UHF, VHF and 800.
Current sites are now experiencing even, as well as odd order harmonics.
Run the standard 2A-B and 2B-A and so forth...Look for anything even closely related to your operating channels.
Third order and even up to fifth order are becoming the norm, especially even order IM products showing up.
If I were on a site with lots of RF on channels above my own, I'd run dual isolators in addition to Lp/Hp filters on the TX side, before the duplexer, but NOT in the antenna line.
[tx]------[amp]----[isolator]--[lpf]----[duplexer]---<antenna feed>

Isolators are IM generators in and of themselves, so placing any filter after them makes engineering sense.
Placing any filter, be it low-pass or high-pass in the main antenna feedline is detrimental to the entire system as it introduces losses by the fact the filters are NOT tuned to pass the desired signal to the receiver.
Same goes for isolators...NEVER place in the RX line. No damage, but NO reception either...

Post by **qball** » Thu May 12, 2011 7:42 pm

I have a little experience with a site that has 800 and UHF at the site. There are two 800MHz frequencies that are exactly 5MHz apart. When they are both transmitting with one of my UHF repeaters, they mix and the sum of the signals equals the input of my UHF repeater. This doesn't so much cause desense, but can cause the UHF repeater's receiver to open up and pass the noise from 800MHz...usually the control channel as it is one of part of the sum.

Try this little intermod calculator to determine when you may have a problem. It helped me find my issue.

http://www3.telus.net/PassiveRF/