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[Rick Earl]

Which raises the question: Are you able to see this directly off-air with your analyzer, or, are you only seeing it through your antenna system?
If it's the latter, it may be an IMD product of something that's getting into your antennas with enough power to create intermod.
A few years ago, I was doing some work for the local arena, and while monitoring their antenna system I was picking up some strong signals just above 700 MHz. When I de-modded them, they were clearly intercom, and it was in fact, the game that was in progress. After determining that there was no legacy 700 MHz com in use, I traced it down to massive IMD in the house antenna system, caused by a BTR TX antenna, that over the years had been turned 90* by the very stiff LMR400 it was connected with. The 90 degree turn pointed it directly at a Shure UA874 antenna which was, at most , 2' away. Those antennas had been set to the +10dB gain position at one point (to drive 25' of cable... ) but I can't recall if I had corrected that by this point in the contract. I have images of both the antenna placement and the resultant trace on the analyzer, but I don't have time to find and post them right now.

My antenna system was direct into a 4 channel Axient receiver.   I set the second BNC set connectors as outputs for cascade and then ran my analyzer from that.  I also never switched to the other antenna.  This is the second time it was an issue, the previous was with a pair of   ULXD Quad receivers with a similar antenna set-up, same cable type, but I think CP Beam Helical antennas were used.  The offending frequency was causing drop-outs, it had not shown up on the WWB scan since it was using the ULX front end to scan.  I had the techs exclude that frequency and re-deploy with new data, I kept the frequency as an exclusion when working in that venue and never got around to trying to figure out what it was.  This time, with continuous  scan, I actually saw it appear, it is intermittent.  I know the house com was off.  (Clear-Com Free Speak).  I had a second analyzer with a LPDA set and ready to go to see if is could figure out a direction, but it did not show up again while I had time to mess with it.  All that to say, I have no idea if it is in the antenna system as I have only seen it through that set-up. Although you all have given me some good places to hunt.   I did test the cables today with tracking generator and they seem to check out OK, at least to my limited knowledge, there was no drop in level across the spectrum.  I have no idea if they are unintentional antennas as well, and how to test that.  I just picked up an Rohde & Schwarz analyzer with tracking generator, but still learning all it's functions.    Again, this is something I can work around, but I would like to find it, so if I encounter something like this again I know what I am dealing with and maybe even how to deal with it.

[Henry Cohen]

I did test the cables today with tracking generator and they seem to check out OK, at least to my limited knowledge, there was no drop in level across the spectrum.

If "there was no drop in level across the spectrum" then something is amiss with the analyzer setup. Coaxial cable always has loss, which is inversely related to frequency and cable length. If you were looking at a minimum of an octave span, you should have seen a gentle slope downward from the start to stop frequencies. The rate of the slope, and the initial loss at the start frequency, is dependent on the coax cable type, condition and connector attachment loss.

Specific coaxial cable losses at frequency vs length is an industry standard specification provided by all reputable cable manufacturers. Generic cable type losses (e.g. RG58, RG8X RG213, RG8) can be found with basic web search (here's one).

[Rick Earl]

If "there was no drop in level across the spectrum" then something is amiss with the analyzer setup. Coaxial cable always has loss, which is inversely related to frequency and cable length. If you were looking at a minimum of an octave span, you should have seen a gentle slope downward from the start to stop frequencies. The rate of the slope, and the initial loss at the start frequency, is dependent on the coax cable type, condition and connector attachment loss.

Specific coaxial cable losses at frequency vs length is an industry standard specification provided by all reputable cable manufacturers. Generic cable type losses (e.g. RG58, RG8X RG213, RG8) can be found with basic web search (here's one).

I would guess there is something amiss with my set-up, as the analyzer is but a week old and I am still learning, I was also only looking at a 25' cable from 500-600MHz as that is where my frequencies fell out.  There may have been a gentle slope, but I would guess I didn't have the resolution set up well enough to see it.

I also forgot to mention in the entire set-up I was using  PWS UHF 470-618 In-Line Filter on each antenna at the receiver.

I am reading how to use the demodulation function and hope to set up later this week and see if I can do some more investigating.

[Henry Cohen]

I would guess there is something amiss with my set-up, as the analyzer is but a week old and I am still learning, I was also only looking at a 25' cable from 500-600MHz as that is where my frequencies fell out.  There may have been a gentle slope, but I would guess I didn't have the resolution set up well enough to see it.

Since your were looking at only about 18% bandwidth, and your scale was probably set to 10dB/Div, you would not have seen any perceptible slope, and you would have minimally seen the fundamental insertion loss of the coax itself, a couble of dB, presuming you calibrated the unit prior to measuring.

I also forgot to mention in the entire set-up I was using  PWS UHF 470-618 In-Line Filter on each antenna at the receiver.

In today's congested, high RF noise floor environment, broadband filters aren't quite useless; they can attenuate strong LMR and LTE on top side and LMR on the bottom, but do nothing for  the adjacent channel high power DTV station. In the case of UHF TV band wireless equipment, 6MHz high Q channel filters are the best option.

I am reading how to use the demodulation function and hope to set up later this week and see if I can do some more investigating.

The R&S can only demodulate AM and FM, if you have that option enabled.

[Loren Miller]

This thread is like reading a geeky Nancy Drew novel...loving it!!!!

Loren

[Rick Earl]

In today's congested, high RF noise floor environment, broadband filters aren't quite useless; they can attenuate strong LMR and LTE on top side and LMR on the bottom, but do nothing for  the adjacent channel high power DTV station. In the case of UHF TV band wireless equipment, 6MHz high Q channel filters are the best option.

I used those for a few reasons.  I knew there were going to be a large number of law enforcement, university security and private security as well as a rescue squad/first aid room about 20' away from on of my antennas, there are also 64 high powered Access Point deployed in the arena, I know they are way out of band, but I figured the filter probably wouldn't hurt.   Although aware of them, I am not experienced enough to start figuring out how to design a system around narrow band filters or how to specify them that fits within our budget.   I would speculate that they are items available for rent, but again, I need to learn a lot more.

The R&S can only demodulate AM and FM, if you have that option enabled.

I do have it enabled, I enjoyed some light reading of the manual and was able to tune in to a local FM broadcast.  Since I don't know what the mystery signal is, I figure I might as well try to listen to it.  I also read up on screen captures and hopefully have enough understanding of the settings  to narrow in my search and see if I can capture the spectral mask.  Luckily the areana is dark the next few days and I had an event postponed, so I may have time to try this week.

[ProSoundWeb Community]