Using separate RX and TX antennas for a repeater

[cpg178]

Currently using a VHF PCTEL MFB1500 series antenna about halfway up a 96ft tower side mounted. I can pick up the repeater on low power (40w out of the rptr) for about 30 miles around, however talk in is limited to a shorter range due to terrain and the height of the antenna on the tower, the added 30-40ft should make a big difference. I do have access to unused antennas with coax pre ran on the top of the tower ranging from some old Phelps Dodge stationmasters to a DB222. Is there any sense in putting my RX up top and keeping the TX where it is, ditching the duplexer in the process? Is SWR need to be dead on for an RX only antenna, I understand it will cut range and performance regardless but depending on how bad it is, will the added 40ft in elevation outweigh that loss? What considerations do I need to take in terms of filtering the RX, there is 2 adjacent towers (~100ft) that each have VHF repeaters on them transmitting 5mhz away from my repeater input, and then about 1/4mile away is a 911 tower with stuff all over the VHF spectrum.

Worst case I might just need to bite the bullet here and get brand new antenna and hardline installed for the entire system to move it to the top of the tower.

 

[kayn1n32008]

If there are close by, inband repeaters, you will probably need band pass filtering on both receive and transmit. At that point, you might as well bite the bullet and use a single antenna at the top of the tower, with a single, continous feedline, not some stuff scabbed together, and use a quality duplexer.

 

[merlin]

A few problems with used antennas like that, one, is they are very tight in their tuning and may not tune to your needed frequecy(s).
At 96 foot, I would think the coax would be 1 inch Heliax or better. Take down the top antenna, and repace with your PCTEL.
Tower top radios will introduce some serious work and configuration so consider that.

 

[prcguy]

At a lot of sites the tower top is reserved for receive only antennas by design. If this is the case then using a top antenna for receive can improve or increase your receive range and the transmit range will stay about the same. On the receive side you would need at least two maybe three 1/4 wave band pass cavities with critical length interconnect cables providing a very narrow pass band at about 1.5dB insertion loss then feeding a low noise low to moderate gain preamp. This is what I ran at the best sites I've ever been at and performance was stellar and noticeably better than running with a duplexer.

In the end you need to calculate or measure exact antenna isolation, then add isolation provided by at least one 1/4 wave pass cavity on the transmitter and the receive cavities. That needs to add up to at least 85-90dB if no preamp is used and if a preamp is used subtract the gain of the preamp from the isolation, meaning you are going to need at least 15-18dB more isolation if a preamp is used. But you will be so happy with the performance you will probably sh*t yourself.

 

[merlin]

Knowing Richardson Tx. do you know of the antenna test site in Murphy ?

 

[prcguy]

Knowing Richardson Tx. do you know of the antenna test site in Murphy ?

No, But Collins used to have a big site and antenna farm in Richardson.

 

[AM909]

No, But Collins used to have a big site and antenna farm in Richardson.

There's towers and some other stuff at CA here and in the field SSW: 32°59'43.4"N 96°39'37.0"W · Richardson, TX

 

[nd5y]

The HF antennas are probably for these and various other government stuff.

FCC Callsign WRN (Shipcom LLC)

FCC Callsign WRN (Shipcom LLC)

www.radioreference.com

FCC Callsign WPXA329 (Rockwell Collins, Inc.)

FCC Callsign WPXA329 (Rockwell Collins, Inc.)

www.radioreference.com

 

[prcguy]

There's towers and some other stuff at CA here and in the field SSW: 32°59'43.4"N 96°39'37.0"W · Richardson, TX

I need to go by there next trip and check that out, its about 3mi from my place in Richardson. Thanks!

 

[AM909]

Limiting a search to active HF, you get the above-mentioned Shipcom 5kW coast station and 1kW flight test licenses, as well as a "define waveforms for high speed data over HF" experimental 25kW license with 24K0J1D emissions, all to Rockwell-Collins.

Expand the search up in freq and include expireds, and you get other interesting stuff, like this log-periodic and other stuff at "ISA", and some other defense-contractor-looking places.

 

[speedway_navigator]

From what I've found VHF isolation maxes out about 60 dB for a vertical difference of 50'. A gain antenna might have more due to the horizontal beam width. See page 88.

https://tacticalrf.com/tech/RR/about_rf_communications.pdf

 

[prcguy]

From what I've found VHF isolation maxes out about 60 dB for a vertical difference of 50'. A gain antenna might have more due to the horizontal beam width. See page 88.

https://tacticalrf.com/tech/RR/about_rf_communications.pdf

Add a 1/4 wave pass can on the tx and rx lines and you can probably add another 30 dB to that. And don’t forget your isolator, you can’t install a repeater in a commercial site without one and you should have one even for a home repeater.

 

[Ubbe]

you should have one even for a home repeater.

I've had several cases where a repeater several 100's meters away that didn't had an isolater for the TX, and has its TX filter wide open to other transmitters in the same band, created intermodulation issues in its power amplifier and at one instance generated a carrier at a repeaters RX frequency that I managed when one pager transmitter a kilometer away where active in the frequency band.

/Ubbe

 

[WRMD298]

If you can run separate TX and RX antennas, it would be best for total isolation between the transmitter and receiver. Furthermore, if you're running a normal 600 KHz split repeater, it is, and always has been very difficult to duplex with appreciable transmitter power without receiver desense, duplex noise or broadband noise generated from imperfect metals on the tower near (or including) your own antenna. You can always keep the duplexer and "split" it - run the RX through one side and TX through the other to separate antennas if a lot of isolation is needed.

You mentioned that you might just buy a new antenna and hardline and move it to the top of the tower. My friend, the top of the tower is no place to be, especially if you're the highest thing around in the immediate area. Every ham in the world seems to want to have their repeater antenna out the very top of the tower (myself included). I have learned the hard way after 25+ years of trial and error that the top most antenna is the most noisy antenna for receive, especially for VHF frequencies and below. I duplexed a repeater for years on a DB224 top mounted at 410 feet. During the dry, windy months, especially during the middle of the day, I had 10 to 15 dB of noise degradation due to dry air static which rendered the repeater useless for marginal/weak stations. The system would also be plagued even more with noise during foul weather (heavy rain, storms, hail, wind, etc). I started to experiment with another antenna down the tower a little ways, a small Diamond X50 at 375 feet. The noise almost went away when operating on the side antenna under ALL of the above conditions.

Since then, I have eliminated the duplexer and moved the repeater's receiver to the lower antenna, and the transmitter to the top. This goes against everything we've been taught - commercial operators always put the receiver antenna out the top and transmitter antennas below. Yes, I know that's the unwritten rule. However, in my situation, receiving on the lower antenna improved overall performance even with a shaded area back through the tower in one direction.

Agreed 100% with the above posts that you need an isolator on the TX whether you use separate antennas or one single antenna, duplexed. If you're using a duplexer, the pass filtering characteristics are usually enough to filter out harmonics that may be caused by the isolator. If you choose to run separate antennas, put the isolator directly on the output of the transmitter, THEN a pass cavity between the isolator and the antenna.

Good luck.

 

[speedway_navigator]

How much distance did you offset your RX antenna from the tower to help reduce any shadowing?
This is from an eastbound exit ramp on I-10 in El Paso, I can't image much signal going or coming from the north.

 

[Ubbe]

That thick pole will always obstruct the signal. It will look like this at different distances:

/Ubbe

 

[WRMD298]

The picture posted above is pretty crazy if anyone needs that antenna to work behind it. Wow.. So, spacing the antenna 1/4 wave off the surface generally results in 3-ish dB of gain in the direction of the antenna, as the mast becomes a reflector. Moving the antenna 1/2 wave off the mast usually results in a somewhat bi-directional pattern, but all of this depends on the size of the mast behind the radiator. Antenna patterns are pretty unpredictable, even with software modeling. I have access to TAP7, which is supposed to be one of the best commercially available tools.

The picture I've attached is what I'm working with concerning my area's primary 2-meter repeater. The DB224 out the top is what I've duplexed as a single antenna for years. It's a 150-160 antenna, and with 480 feet of 7/8" line I get about a 1.6:1 match in the building at the TX frequency. Yes, I know the match throws the duplexer curves off a little and I've compensated for that. I've tried a variety of duplexers and filters, from a 4-cavity Sinclair Q2220E to a hefty Wacom 6-cavity with 10" cavity diameters and 2.5 dB of loss. I've also tried running high (>100W), medium (50W) and low (25) power settings with various results. The repeater is either a Quantar 125W or a 40W MTR2000. Sometimes the top antenna duplexes fine, other times, any amount of power creates what appears to be broadband "illumination" noise. Depending on the time of day, humidity, weather conditions (especially dry air) I get up to 3 microvolts of receive noise (that's straight receive, NOT desense from the transmitter!). It's the highest thing on the tower and the highest thing is most always going to be the noisiest! I did some further searches on the subject, and I'm not alone in these findings! Check the link below. Good article:

Pecipitation Static or P-static

Scroll down to "Case 4 - repeater antennas"

If the wind is calm and the humidity is up, the noise floor is relatively low and I can duplex full power into the antenna with zero problems. But 75% of the time, this is not the case.

The Diamond X50 is my spare antenna. To answer the question above, it's mounted 6 feet away from the tower, but the tower face is 30 inches and there's a lot of metal up there. There is some shading through the tower, as to be expected, but it's not a deal-breaker to use it.

My initial thoughts were to receive on the top antenna and transmitting bunches of power on the side mounted X-50, attempting to gear my performance toward the receive side of things like commercial operators do (such as the 800 MHz antennas in the photo). While the amount of isolation between the two antennas is high enough to run the repeater without filtering, I tried this and it worked - but the general noise floor on RX was still very high most days. I dared to do something unthinkable - RECIEVE on the lower antenna, and TRANSMIT on the top. Guess what - the Diamond X50 had 10 to 15 dB LESS NOISE than the top antenna during adverse conditions. Although the feedline loss, antenna gain and the null in one direction are all disadvantages to using the X50 as my primary receive antenna, the reduced noise floor improvement offsets the other factors by quite a margin. I have been running this configuration for about a month and the change was certainly an improvement.

I am fairly convinced that on VHF frequencies and lower, the only antenna that needs to be out the top of the tower is a paging antenna where transmit is the primary concern. At UHF frequencies, I have never really seen degradation on RX by being out the top. But real-time experience has taught me that side mounting VHF arrays for receive are probably (likely) the best way to go for the lowest ambient noise on RX.

 

[prcguy]

If it is wind blown noise of some sort then there should be no difference having the antenna at the top of the tower vs further down, they will all get the same wind with precipitation or dust or whatever is contributing to the noise. You are using an all metal grounded antenna so static buildup should not be a problem and I think your noise is from something else.

The top of the tower is always the best spot for rx antennas based on handhelds and low power users where they will always hear the repeater but not always get into it with a handheld. I think more research is needed to find the exact cause of your noise and try to eliminate it.

 

[WRMD298]

Roger on all of the above. I (was) right there with you on trying to find the noise source for several years now until I tried the lower antenna. At the site, the only other systems there are a 151/159 MHz high band repeater (on the DB224 also side-mounted at 375') and an 800 MHz trunked system. The tower lighting system is a Drake LED. I have (numerous times) during elevated noise floor turned the entire 800 MHz trunking system OFF, turned the other VHF high band repeater OFF, turned the tower lighting system OFF, disconnected power from the building and furthermore disconnected the facility UPS, 800 MHz tower top amplifier and went even a step further and disconnected the complex power at the meter base, then powered my receiver and test equipment with a battery. Even with all of this, the same amount of noise was present on the top antenna while the lower DB224 and the Diamond X50 were both fairly quiet. A second scenario on a smaller tower, 100 feet of Rohn 25G, about 30 miles away and located at a facility in seriously "no-where" land has two DB222 VHF high band antennas. One is right out the top, and the other, mounted down around the 80 foot level. Both are DC grounded antennas, which DO eliminate static buildup on the center pin, but do NOT eliminate RF noise caused BY the static in the RF spectrum at the receiving frequency. I was so curious to my findings at the previous site that I tested for RF noise on a dry, windy day at site #2 and sure enough, the top antenna was 10 dB noisier, on average, than the lower DB222. This site has zero pole-mount transformers for nearly half a mile and the utility entrance is buried. The users of this particular system really don't care about 10 dB of elevated noise floor because it only needs to reach 10 to 15 miles out and everyone transmitting back into the system is at a -70 or better.

I am not opposing the idea that the high noise floor can be coming from somewhere else, but again, this primarily happens on dry, windy days when the humidity is 40% or lower. I have also been caught at the site when a storm was many miles away and noticed the same phenomenon with noise. I would like to test more sites to maybe solidify the theory that the top antenna is always the noisiest, especially on VHF high band, but this would take much time and effort just to satisfy the curiosity (sometimes worth the effort).

 

[WB5UOM]

I cant agree with the "top antenna has the most noise". I have multiple,as in many DB-224's top mounted on 400 +foot towers and and able to add preamps to the receivers and still not problematic noise.
Got to be something else.