A UHF Milair Search Problem Solved!

[chgomonitor]

(These observations are based purely on local, practical application and not on lab measurements. My listening post is in a high RF environment - suburban Chicago).

A long time ago I settled on the Uniden BC780XLT for use as a dedicated Milair (mostly 225 - 400 MHz) scanning and searching receiver. My personal experience has shown them to be highly sensitive and reasonably selective radios for this purpose, especially for long distance and weak signal reception. (Oddly, the discontinued Radio Shack Pro-2045 running a close second). Virtually all of my local Milair monitoring is aimed towards locations and activities quite far from here as there isn’t much truly local activity.

I push the front ends of most of my radios pretty hard, employing a Diamond D-130J Discone on a 30 foot outdoor mast fed with heliax into a Stridsberg Engineering MCA208 active, 8-port multicoupler:

http://www.stridsberg.com/mca208a.htm

A number of nearby FM broadcast stations cause tremendous VHF air band noise, so I use a Winegard FT7500 trap in front of the multicoupler to kill most of that:

https://www.tselectronic.com/winegard/ft7500.html

I’m a long time Milair monitoring buff. I found the recent loss of access to flight supplement information particularly depressing. Then I began seeing reports of local UHF Milair frequency changes at places like Selfridge and became increasingly concerned about other possible local flying activity on new frequencies.

I started thinking about the techniques we used long ago to find active Milair communications. Things like computer driven radios, search and store and recording audio on suspect frequencies. I fired up all my old tech systems to see how they’d do in today's listening environment. Search and store or computerized logging using the BC780XLT seemed to win hands down. But I found I had a serious new challenge.

Over the course of two months of almost non-stop automated searching using two 780’s I discovered I had done a really great job of logging a ton of local noise, even with the 780’s attenuator set to “ON”. The local FM broadcast stations signals were mixing with high power, local UHF repeaters, automated VHF taxicab transmitters, NOAA weather radio and 800 MHz Nextel noise. This created a wideband wall of noise which was drifting around 300 - 400 MHz, basically flooding the front ends of the 780’s. Not so good. Thinking about the problem and what we used to do about it, I recalled my departed, dear old friend Jeff Zeman and a place he did a lot of business with:

http://www.minicircuits.com/

(I suspect Jeff’s monitoring ghost will haunt me forever!)

After a week of pouring over spec sheets and web studies on things like bandpass, notch and cavity filters I settled on the idea of using a combination of high and low pass filters in series in the antenna line feeding a 780XLT. The idea was to create my own custom bandpass filter for 225 - 400 MHz at minimal cost. Though I would have preferred something with a bit steeper UHF cut-off, I finally decided on these two units from minicircuits:

Low Pass Filter BLP-450+ Spec Sheet:
http://www.minicircuits.com/pdfs/BLP-450+.pdf

High Pass Filter BHP-250 Spec Sheet:
http://www.minicircuits.com/pdfs/BHP-250.pdf

The total was about 80 dollars. Five days after web ordering they arrived and I got to work this morning. I simply connected one to a multicoupler output, the other to it and finally the 780 antenna coax jumper to the end.

These don’t create as much of an “RF wall” as I had assumed they would. A few high power local VHF stations are still coming through, as are most wide-area high power UHF stations, albeit much weaker. Everything below 150 MHz is just about totally gone however, including the FM broadcast band in its entirety and my local airport ATIS on 124.200.

So, for the past few hours I’ve been searching 225 - 400 MHz with the now bandpass filter equipped BC780XLT. Wow! Total success! The only noise I’m left with is from the local oscillators off of other radios in the shack and sensitivity overall seems very greatly improved. I caught some low altitude guys flying in the Hilltop / 12 Mile MOA’s for the very first time and this is some 140 miles away across the flat lands of the Midwest. Best of all, I’m finally able to turn off the attenuator on the 780 and I’m now enjoying the true sensitivity offered by this fine radio while performing a wide band search. I’m a very happy camper and I’ll be resuming my automated search and store routines on UHF Milair.

If you have a similar local noise UHF Milair situation working against you I’d encourage you to try this or a similar solution. Even at fairly quiet monitoring locations I’m guessing you’ll see some improvement in 225 - 400 MHz sensitivity. It really makes a world of difference, especially for weak signal work.

Happy Scanning! - Ted

 

[FLANO]

Thanks Ted,

I think I'll have to give this a try. I use 2 PRO-2006's for most of my MILAIR activity. It'll be interesting to see what it would do for me, as I've got a number of ranges and MOA's that I routinely hear already here in Tucson.

 

[DPD1]

I’m a long time Milair monitoring buff. I found the recent loss of access to flight supplement information particularly depressing. Then I began seeing reports of local UHF Milair frequency changes at places like Selfridge and became increasingly concerned about other possible local flying activity on new frequencies.

I've never worried much about this, because for ATC you're eventually going to hear them repeat a switch. So even if something changed, it wouldn't take long to find. The tactical stuff is always going to change no matter what, but that's OK... I think it would be boring if everything stayed the same all the time. And if some people are put off by that because they want everything all listed out for them in a book, then they should probably stick to police and fire. I think mil stuff is more for the puzzle lovers.

You might also limit unwanted signals with band specific antennas. Except in the case of VHF if you have a VHF noise, then it might get worse. I feel your pain on that because I live in an extremely hostile RF environment and VHF is destroyed to the point that I gave up trying to fix it. You can't go all the way through UHF without blocking at least 100 freqs around here.

Dave
www.DPDProductions.com
Antennas & Accessories for the RF Professional & Radio Hobbyist

 

[chgomonitor]

Changes

I've never worried much about this, because for ATC you're eventually going to hear them repeat a switch. So even if something changed, it wouldn't take long to find. The tactical stuff is always going to change no matter what, but that's OK... I think it would be boring if everything stayed the same all the time. And if some people are put off by that because they want everything all listed out for them in a book, then they should probably stick to police and fire. I think mil stuff is more for the puzzle lovers.

You might also limit unwanted signals with band specific antennas. Except in the case of VHF if you have a VHF noise, then it might get worse. I feel your pain on that because I live in an extremely hostile RF environment and VHF is destroyed to the point that I gave up trying to fix it. You can't go all the way through UHF without blocking at least 100 freqs around here.

Dave
www.DPDProductions.com
Antennas & Accessories for the RF Professional & Radio Hobbyist

Hi Dave and Thanks. My situation here is a little different as I can't hear virtually any of the ground transmitters since I'm no where near an active base. My main target is the Volk CRTCC up in Wisconsin, over a hundred miles away as well as some refuelling tracks and other MOA's and airspace and routes. Activity is sparse, to say the least. Not at all unusual to go a week or more with no traffic. But when Volk heats up it does sometimes get pretty interesting. Basically, I'm grateful for any little snippet I can catch, let alone the readbacks, LOL. Any oh yes, it's always a search and puzzle. You're right, that is part of the fun - but these days I'll take all the reference help I can beg, borrow, steal or scrounge.

Funny you replied as I've actually been considering your Milair beam antenna, which I understand is a log periodic. I've also been thinking about building a somewhat wideband Yagi cut for around 312 MHz. Issues for me are "omnidirectionality", tower space and cost. I would then take this particular receiver and get it off the discone / multicoupler and onto it's own coax feed.

The other thing I've been thinking about, though I'm not a really a preamp kind of guy is this from minicircuits:

http://www.minicircuits.com/pdfs/ZX60-33LN+.pdf

Gain seems a tad high but that low nose figure is really wild. Ah, probably just make all the intermod a lot worse, LOL.

Happy Scanning! - Ted

 

[chgomonitor]

Tucson / 2006

Thanks Ted,

I think I'll have to give this a try. I use 2 PRO-2006's for most of my MILAIR activity. It'll be interesting to see what it would do for me, as I've got a number of ranges and MOA's that I routinely hear already here in Tucson.

We toured the Tucson and Phoenix listening environments this past Spring. A lot to listen to, not like here. Consider yourself fortunate to live near active areas. The 2006 is a venerable receiver. I used them for many years and still have one in the shack, with an Opto board installed. I suspect it's front end and selectivity is actually better than the 780, but the 780 is more sensitive.

Noise and front end overload are probably less an issue for you as a result. I'd suggest opening the squelch while searching 225 - 400 with a 2006 and manually stepping through the band, especially 300-400. If you find areas where you are hearing a lot of background hash and noise you'd probably benefit from some sort of bandpass filter setup.

 

[DPD1]

Hi Dave and Thanks. My situation here is a little different as I can't hear virtually any of the ground transmitters since I'm no where near an active base. My main target is the Volk CRTCC up in Wisconsin, over a hundred miles away as well as some refuelling tracks and other MOA's and airspace and routes. Activity is sparse, to say the least. Not at all unusual to go a week or more with no traffic. But when Volk heats up it does sometimes get pretty interesting. Basically, I'm grateful for any little snippet I can catch, let alone the readbacks, LOL. Any oh yes, it's always a search and puzzle. You're right, that is part of the fun - but these days I'll take all the reference help I can beg, borrow, steal or scrounge.

Funny you replied as I've actually been considering your Milair beam antenna, which I understand is a log periodic. I've also been thinking about building a somewhat wideband Yagi cut for around 312 MHz. Issues for me are "omnidirectionality", tower space and cost. I would then take this particular receiver and get it off the discone / multicoupler and onto it's own coax feed.

The other thing I've been thinking about, though I'm not a really a preamp kind of guy is this from minicircuits:

http://www.minicircuits.com/pdfs/ZX60-33LN+.pdf

Gain seems a tad high but that low nose figure is really wild. Ah, probably just make all the intermod a lot worse, LOL.

If you live in a generally RF quiet area, the pre-amp could possibly work well. If there was a way to easily turn it on and off, you could use it selectively. Or like you said, it could make a lot more problems. It's hard to know for sure.

If you're trying to hear distant stuff in one general direction, then yes, the UHFLP would probably be good for you. You would still get general omni ability. The problem with yagi in that situation is that it's way more narrow banded. You would have heavy drop-offs as you go out from that center. It would also would have bigger nulls to the sides than the LP, decreasing your omni ability.

Dave
www.DPDProductions.com
Antennas & Accessories for the RF Professional & Radio Hobbyist

 

[prcguy]

The mini-Circuits filters have generally very poor skirts, maybe 6dB/octave and you need something with really good attenuation right above and below the UHF air band. Check Ebay item # 160259152454, which is listed as a 225-400Mhz low pass filter. This may be a bandpass but at the very least it will chop everything above 400 as a start. I've found great 225-400MHz band pass filters on Ebay for cheap with excellent skirts (40dB down within 10Mhz of the edge) and lots of other useful filters. Here is an example of what to look for.
prcguy

 

[zguy1243]

Mike and all,

That ebay auction for that filter is suspect. I bought that exact filter from that guy 3 weeks ago and never got a thing from him. He wont answer my emails at all either. It looks like a promising filter but I would stay away from that seller.

The mini-Circuits filters have generally very poor skirts, maybe 6dB/octave and you need something with really good attenuation right above and below the UHF air band. Check Ebay item # 160259152454, which is listed as a 225-400Mhz low pass filter. This may be a bandpass but at the very least it will chop everything above 400 as a start. I've found great 225-400MHz band pass filters on Ebay for cheap with excellent skirts (40dB down within 10Mhz of the edge) and lots of other useful filters. Here is an example of what to look for.
prcguy

 

[chgomonitor]

Filters

The mini-Circuits filters have generally very poor skirts, maybe 6dB/octave and you need something with really good attenuation right above and below the UHF air band. Check Ebay item # 160259152454, which is listed as a 225-400Mhz low pass filter. This may be a bandpass but at the very least it will chop everything above 400 as a start. I've found great 225-400MHz band pass filters on Ebay for cheap with excellent skirts (40dB down within 10Mhz of the edge) and lots of other useful filters. Here is an example of what to look for.
prcguy

You are probably quite correct. I tend to shy away from most surplus gear, and ebay stuff especially LOL - though I must admit that Bird unit in the pic looks very tempting, indeed. I did search ebay at the time and didn't see anything I really liked. A lot of other stuff I found is really built for transmitting through and I didn't see the sense in paying extra for capabilities I wasn't going to use. The minicircuits units do have less than perfect drop-offs, but what can I say - it worked for me. Thanks anyway.

 

[DaveNF2G]

On-channel or in-band interference can't be blocked without losing the desired signals. Some stuff will always get through.

As to frequencies (non ATC), I often hear exchanges of AR and other frequencies between flight leaders when they are preparing for exercises.

 

[chgomonitor]

Noise

On-channel or in-band interference can't be blocked without losing the desired signals. Some stuff will always get through.

As to frequencies (non ATC), I often hear exchanges of AR and other frequencies between flight leaders when they are preparing for exercises.

That is true, assuming the the interference is "real" - actual in-band RF presence. But 99 percent of my problem relates to the receivers themselves - their front ends simply being unable to reject locally high RF levels in adjacent bands. This is also why one is always better off in a locally quieter listening environment. Strangely, you are also sort of conversely correct - you can't get rid of adjacent band overload without losing at least a little sensitivity on the target band. This was described in most of the papers I read on bandpass filters. You'll always introduce a tiny bit of loss even in the target band. The reason is these types of devices aren't perfect. Plus they have some insertion loss, too.

The bandpass filter(s) create sort of a crude pre-selector. The radio's front ends no longer go into overload mode while trying to sort out and reject out of band interference. I figure I've probably introduced one or two DB's of loss in-band with the filters. But by releasing the front ends from their overloaded state the radio is now able to perform as it was designed using it's full sensitivity. My overall result was an almost complete elimination of interference and an unexpected dramatic boost in sensitivity. Your local results will likely differ.

Happy Scanning! - Ted

 

[chgomonitor]

The mini-Circuits filters have generally very poor skirts, maybe 6dB/octave and you need something with really good attenuation right above and below the UHF air band.

Actually, as near as I can read the charts, my low pass filter has about 10Db cut-out at 460 MHz and more above that. This is where most of my local UHF overload resides, and it seems a fairly effective solution.

The high pass filter doesn't really to start to bite in until around 200 MHz. That concerned me more, as I do have a local television station between 200 and 210 MHz. But 99 percent of my local VHF offenders are below 200 MHz.

 

[prcguy]

The filters I pictured in another post have about .3dB loss each within the pass band.
prcguy

That is true, assuming the the interference is "real" - actual in-band RF presence. But 99 percent of my problem relates to the receivers themselves - their front ends simply being unable to reject locally high RF levels in adjacent bands. This is also why one is always better off in a locally quieter listening environment. Strangely, you are also sort of conversely correct - you can't get rid of adjacent band overload without losing at least a little sensitivity on the target band. This was described in most of the papers I read on bandpass filters. You'll always introduce a tiny bit of loss even in the target band. The reason is these types of devices aren't perfect. Plus they have some insertion loss, too.

The bandpass filter(s) create sort of a crude pre-selector. The radio's front ends no longer go into overload mode while trying to sort out and reject out of band interference. I figure I've probably introduced one or two DB's of loss in-band with the filters. But by releasing the front ends from their overloaded state the radio is now able to perform as it was designed using it's full sensitivity. My overall result was an almost complete elimination of interference and an unexpected dramatic boost in sensitivity. Your local results will likely differ.

Happy Scanning! - Ted

 

[WartHog]

I'm glad to hear this radio is so sensitive on the Military band, it's a shame I find it horrible up in the 900 meg range. So poor in fact, that search mode is useless up there, as even on a fairly strong but weak signal it will never lock on. It is so poor in this area, that even if you plug in all your favorite active frequencies in this range, even with signals present, the dang radio will never open the squelch EVEN IN SCAN MODE with the squelch set as absolutely tight as you can set it. You can even unmute the speaker, and scan or search, and hear in the background active chatter, yet the scanner will never stop on a frequency. Now that's poor!

I do find this a good radio for military aero also. It is also very good down at 30 to 50 megs.
Too bad 900 megs is like not even being there.