Pages: [1] 2   Go Down

Author Topic: UHF mobile duplexers, rang and performance?  (Read 1871 times)

KK6RPX

  • Member
  • Posts: 190
UHF mobile duplexers, rang and performance?
« on: February 09, 2019, 06:37:46 AM »

Hi All,

I have been fooling around with a 5 watt UHF repeater and a mobile duplexer. All I can get for range is less than 1/4 mile. I think de-sense is the issue. I have tuned the duplexer with a spectrum analyzer and am getting around 70dB of rejection. Is this normal or am I doing something wrong?

Thanks!
Logged

K5LXP

  • Member
  • Posts: 6823
    • homeURL
RE: UHF mobile duplexers, rang and performance?
« Reply #1 on: February 09, 2019, 07:03:45 AM »


Very well could be.  Do you have any way to test for desense?  Fairly straightforward on the bench with a service monitor or signal generator.

That would be the first test, if that didn't pan out it could be something out of band causing desense when connected to an antenna.  You didn't say where this thing was located but "mobile duplexers" are notch only and at a congested radio site any other transmitters will be present at the RX port.

Mark K5LXP
Albuquerque, NM
Logged

KK6RPX

  • Member
  • Posts: 190
RE: UHF mobile duplexers, rang and performance?
« Reply #2 on: February 09, 2019, 07:32:42 AM »

Mark,

Thanks for the reply!

The repeater is located in my shack at the moment so not in an RF heavy environment. It is connected to a Comet GP-9 antenna with the top of the antenna at about 65'. I do have a spectrum analyzer with a tracking generator but I just got it a couple of weeks ago and am clawing my way up the steep part of the learning curve. Could you describe how to test for desense?



Logged

W9IQ

  • Member
  • Posts: 8866
RE: UHF mobile duplexers, rang and performance?
« Reply #3 on: February 09, 2019, 10:22:03 AM »

The first step is to determine how clean is your UHF transmitter. If your receiver has a 0.25 uV sensitivity, for example, and your 5 watt transmitter is putting out phase noise on the receiver frequency that is -150 dBc, it will desense the receiver regardless of the effectiveness of the duplexer.

What are you using as the receiver and transmitter for your repeater?

Are you using double shielded coax for all your interconnects?

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KK6RPX

  • Member
  • Posts: 190
RE: UHF mobile duplexers, rang and performance?
« Reply #4 on: February 09, 2019, 11:38:56 AM »

Thanks for the reply.

A little background: I am a fairly new ham and am learning what I need to know to take over maintenance for a pair of repeaters in a rural area when the owner goes SK. Which hopefully won't be for a long time but he is 86. Electronics has been my hobby since I was a kid but being self taught I have lots of gaps in knowledge. To the end of learning what I need to know I purchased a Rigol DSA815TG a couple of weeks ago and am learning how to use it. The repeater is a TKR-830. No, I am not using double shielded coax jumpers. I will order some RG-400 soon but I am still recovering from the financial hit from the analyzer. I do have a directional coupler that I got as well.

With that said, I just took a look at the repeater output by draping a coax jumper over the antenna feed line and keying up the repeater. I can see that it is a little off frequency, maybe a few kHz. I'd like to learn how to properly test the output. For starters, with an output of 5 watts and a maximum input power of 20dBm, my calculations show I would want to put 36.9dB worth of attenuation inline. I have a Texscan variable 1-110 dB 1-10dB/step attenuator. I have been unable to find the exact specs of this unit but I think it should have no problem with 5 watts for short periods. Of course, I can set the output power of the repeater down as well.

So please, school me and I'll and go and try to follow instructions. I am having a blast playing with the analyzer and want to learn!

EDIT: The TX frequency is pretty close to 1kHz low.
« Last Edit: February 09, 2019, 11:55:39 AM by KK6RPX »
Logged

W9IQ

  • Member
  • Posts: 8866
RE: UHF mobile duplexers, rang and performance?
« Reply #5 on: February 09, 2019, 12:12:40 PM »

Jon,

Don't connect your attenuator as a load for your 5 watt transmitter as it probably cannot handle the power dissipation.

Set your SA to monitor a frequency range from the transmit frequency to the receive frequency. Turn off the TG. With your draped coax setup, see how many dBs of difference there is between the transmitter output frequency and the receive input frequency when the transmitter is keyed.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KK6RPX

  • Member
  • Posts: 190
RE: UHF mobile duplexers, rang and performance?
« Reply #6 on: February 09, 2019, 01:50:34 PM »

Glenn,

Sorry for being thick but I am not exactly sure what you want me to do. By draping the coax I can see the output of the repeater but how can I see the input attenuation?

What I just did was connect an antenna to the input side of the duplexer, a dummy load on the TX side of the duplexer and keyed up a handheld on the input and output frequncies and they seem very close. Of course, local coupling into the coax etc. may be the issue, it is a fairly strong signal. I'll see about getting the XYL to go key up further away while I watch the analyzer.

Thanks!
Logged

KK6RPX

  • Member
  • Posts: 190
RE: UHF mobile duplexers, rang and performance?
« Reply #7 on: February 09, 2019, 02:23:36 PM »

A little more info...

I hooked up the RX and TX sides of the antenna to different antennas. There is about 45' of vertical and 120' of horizontal separation. I got about the same result as I did using the duplexer, a little more but barely noticeable. So maybe it isn't the duplexer and desense that is the issue.
Logged

KA5IPF

  • Member
  • Posts: 1824
    • homeURL
RE: UHF mobile duplexers, rang and performance?
« Reply #8 on: February 10, 2019, 04:17:23 PM »

If you are determining the repeater is off freq using the spectrum analyzer that could be the problem. It doesn't have a great timebase, it's more or less a ballpark deal. You need to check frequency with a known calibrated frequency counter. For grins hook up the SA to an outside HF antenna and check the frequency of WWV if it will receive it.
Logged

KK6RPX

  • Member
  • Posts: 190
RE: UHF mobile duplexers, rang and performance?
« Reply #9 on: February 11, 2019, 07:24:26 AM »

I did just that and it seems pretty close. First use of the demod function, took a minute or two to get it working. I didn't spend a lot of time dialing it in, I have to get ready to go to work for a 48 hour shift. Of course, being close at 10MHz doesn't mean all that much, if the error PPM is low it won't add up to much of an error by 10 MHz.

Still hoping for some advice as to how to figure out why this repeater isn't working well. The questions are:

Should I be getting better performance from the mobile duplexers?

Does the separate antenna test above indicate that it isn't de-sense or is the separation between the antennas insufficient?

Above K5LXP mentioned above it is easy to test for de-sense with a SA and SG which I have. Anyone have any more information about that?

Thanks!
Logged

DEANQ

  • Member
  • Posts: 13
    • homeURL
RE: UHF mobile duplexers, rang and performance?
« Reply #10 on: February 11, 2019, 07:37:39 AM »

What are you using as the receiver and transmitter for your repeater?
Are you using double shielded coax for all your interconnects?
- Glenn W9IQ
Do NOT dismiss your interconnecting coax. ANYTHING other than double shielded coax, that close to your xmitter/reciever, will cause problems. JMHO
Logged

K4JJL

  • Member
  • Posts: 1194
RE: UHF mobile duplexers, rang and performance?
« Reply #11 on: February 11, 2019, 07:51:49 AM »

What are you using as the receiver and transmitter for your repeater?
Are you using double shielded coax for all your interconnects?
- Glenn W9IQ
Do NOT dismiss your interconnecting coax. ANYTHING other than double shielded coax, that close to your xmitter/reciever, will cause problems. JMHO

Also, don't use LMR-XXX for full duplex operations.  It's very noisy.
Logged

KK6RPX

  • Member
  • Posts: 190
RE: UHF mobile duplexers, rang and performance?
« Reply #12 on: February 11, 2019, 11:01:24 AM »

So, I have been looking up testing desense. Basically I want to find the baseline RX sensitivity of the receiver and then key up the transmitter and test again. Subtract the first reading from the second and I should have the amount of desense.

To that end, I put my variable attenuater inline between the SG and the receiver and the RX antenna input and set the SG output to -20dBm. At first, even with the attenuator at 110dB I was lighting the RX busy led. I quickly realized that it is indeed a problem using regular coax as it was bleeding through. I set things up to keep better spacing between all the coax and then it dropped to about 30dB on the attenuator for about a total of -50dBm before the busy light stopped lighting.

I also found that have the shield of the coax connected lowered the sensitivity of the receiver. Meaning that if I unscrewed the outside of the PL259 connector and left just the center conductor connected I got better sensitivity. Any thoughts on why that would be?

And yes, I realize that to get meaningful results I need to get better coax. I am ordering some, it is just that I have been spending too much on radio lately. I'll get some.

Thanks!
Logged

KK6RPX

  • Member
  • Posts: 190
RE: UHF mobile duplexers, rang and performance?
« Reply #13 on: February 11, 2019, 11:24:44 AM »

I should have mentioned that I had the SG set to the RX frequency and the span to 25kHz. Is that what I want to test sensitivity?
Logged

WB8VLC

  • Member
  • Posts: 1155
RE: UHF mobile duplexers, rang and performance?
« Reply #14 on: February 11, 2019, 01:02:06 PM »

What, What????   you said>>>I also found that have the shield of the coax connected lowered the sensitivity of the receiver. Meaning that if I unscrewed the outside of the PL259 connector and left just the center conductor connected I got better sensitivity. Any thoughts on why that would be?

This is similar to the conversation below from the movie the SANDLOT:

Squints:
Where did your old man get that ball?

Smalls:
I don't know. Some lady gave it to him. She even signed her name on it... Ruth. Baby Ruth.

All:
BABE RUTH?!

(Everyone screams in horror and rushes to the fence)

Smalls:
I was gonna put the ball back.

Squints:
But it was signed by Babe Ruth!

Smalls:
Yeah, you keep telling me that! Who is she?

Ham Porter:
WHAT?! WHAT?!

Kenny:
The sultan of swat!

Bertram:
The king of crash!

Timmy:
The colossus of clout!

Tommy:
The colossus of clout!

All:
BABE RUTH!

Ham Porter:
[emphatically] THE GREAT BAMBINO!

Smalls:
[in shock and horror] Oh, my God! You mean that's the same guy?!



You are killing me Smalls,

You have serious cable issues not to mention the lack of other pieces of test equipment needed to test your system.


Once you get your interconnect cable issues resolved you then need to perform an 'Effective Sensitivity Test' with everything connected up as follows.

To properly check effective sensitivity you need either a COMM test set that can run in duplex mode which injects a RX signal while simultaneously measuring TX output power.

In lack of this you can use a seperate RF generator coupled into a directional coupler (DC) that is connected at the duplexer output  port in line with the coax going to the antenna.

If you go the DC route then you also need to either measure or accept the coupler Mfg.s coupling value in dB, let's just say the Mfg spec's his DC at -30 db for instance on the port that the RF sig will be injected into the repeaters duplexers antenna port.

Also be aware that the TX power out of the duplexer will appear at the RF gens output port minus the directional couplers (DC) coupling in dB.

In our example the full TX power will be 30 db lower than the true tx level so make sure that your Rf gens output port can take this TX level.

For instance if you are putting out +40 dBm at the duplexers ant port then the -30 db DC coupling number will let +10 dbm of tx power appear at the RF gens output port, if your RF gen cannot accept +10 dBm then you need added external attenuation on the DC port coupled in to the duplexers input port until the Rf gen level is safe.


You need to then set your RF gen to the repeaters receive frequency and set the RF gens AUDIO for 3 KHz FM dev with a 1KHz test tone, if your repeaters RX uses CTCSS then you will need a second audio gen set to the repeaters RX CTCSS tone with a Dev between 500 and 750 hz.

Connect the RF gen to the DC so that the sig is coupled into the duplexer antenna port and not towards the antenna.


Next begin with a RF gen level around -80 dBm.


Next increase or decrease the the RF gen level until you have a nice 12 db sinad reading coming out of the repeaters receiver. 

Take the RF gen level above and add it to the directional couplers (DC) port loss, -30dB in our example and you have your effective sensitivity number.


IE for example if your Rf gen level is -70 dBm for ~12 db sinad for the above test and your DC has -30 dB on the port coupled into the duplexers antenna  port then your effective sens is (-70) + (-30) or  -100dBm.


 A COMM test set such as an HP8920 set up in full duplex mode takes care of all of the above and makes RPTR eff sens tests fast and easy, your little spectrum analyzer with the built in tracking gen isn't made for this type of test as I don't think that it has the proper audio generators and such.
Logged
Pages: [1] 2   Go Up