Batboard

This may or may not sound odd but does temperature make a difference in reception? - i would say so after what i'm experiencing. I'm running multiple roof top antennas to my motorola astro units and reception is always decent but when it's cold (it has to be atleast 20 degrees or colder outdoors) and my reception drastically improves. It's about a 100 foot run. I was told that when it's cold like that the electrons run easier through the coax with less resistance and less loss...is that true? Would 80 degrees in temperature (summer/winter months) change even make a difference?

Also for anybody that knows what would cause this....is there a way to make it so that even when it's above the 20 degree mark i can still receive what I can when it's cold? Better coax? Amps?

Thanks for any help.

Unless you have some weird water in the connector problem that cures when it's frozen outside, I'd say you are more likely experiencing atmospheric ducting than "super conductor" coax. I'd do a prop study with Radio Mobile between your house and your preferred systems to see what's going on. Might be time to really scrutinize your antenna and coax system too.

I've posted this on the radioreference board as well and I've gotten replys saying it could be water that is freezing somewhere in the coax/connectors. I guess i wouldn't say my reception gets a ton better but the systems that are very weak and if you were to look at a map, are just on the boarder of my reception line..those will come in loud and clear when it's 20 degrees or less. If it's warmer than 20 it won't even break squelch. Very strange.

What condition is your antenna system in? Is it relatively new with single runs of good quality cable and connectors, or is kinda pieced together with what you could afford, and what you had around to work with?

By your description you are working the fringe and if you had 3 to 6db more you could receive the systems better. 10db swings in rssi are normal for rx paths obscured by trees. Are you surrounded by forests, and if so, what species? Are your antennas well below the canopy, or do they have some elevation above it? What band are you listening to?

Bill, thanks for the replys!

I'm using four ASTRO spectra's VHF analog and P25 for receive only monitoring. I'm using pretty basic RG-6 coax. I just replaced all my old coax which was RG6 as well with new a few weeks ago b/c what I had was pretty old. There was no difference in the RX when I replaced the old - that was another reason for replacing it because I thought maybe it was the old coax/connectors. It is 75ohm coax but for receiving only i've read that was okay and i've been using it for years with no damage or anything done to my radios. I do have some trees around me but the antenna system is pretty high and i'm located on a hill. My elevation here is about 1200 feet.

Here is a photo of my antenna set up:

I've also purchased these a few months ago: http://cgi.ebay.com/MOTOROLA-BDA-S4-SIG ... 3efe0fc678 - I was using splitters before and these work very well and prior to using these I was not getting those distant systems..regardless of temperature.

So whatever is happening now (when it's below 20 degrees) it's giving me just enough more dB's that I can receive them.

Nice setup. Some would argue about the impedance issues of using 75 versus 50 ohm cable for receive, and they may even be able to prove on paper you'd lose or gain a few db if everything were perfect. But, that wouldn't immediately explain the apparent improvement you're experiencing when it's below freezing unless the cable characteristics changed significantly which is doubtful. +20F is not that cold, and definitely much hotter than where super-conducting properties would be revealed if they existed in that cable. Since it's a new installation with the same performance you experienced with similar cable previously, we can probably rule out wet connectors. You didn't say if you replaced the antennas or not. I'll assume you reused your existing antennas. In my experience a wet colinear antenna is a dead stick. They tend not to work very well. We can probably rule those out as well.

I see three antennas. You talk about four radios, splitters, and some amplifiers. Tell me how this is all hooked up. Does each radio have it's own amp and stick, or is one of the sticks pictured going to a four port amp and then to the radios? Is that amp outside? If so, is it possible the amp is benefiting from the cold?

Let's go back to my idea about ducting, but look at it differently. Without the amps, you say you can't receive the desired systems at all. With the amps you still can't receive them during warmer weather, but once it gets below freezing you can. You have good elevation. Do you have line of sight to these systems, are they beyond the horizon, or are they obscured by terrain? Is freezing weather normal in your area? When it's freezing, do you have dense cloud cover, clear skies, or does it matter - that you are always guaranteed receiving these stations once it freezes? I'm betting you only receive the far stations during cold clear conditions - that the atmosphere is reflecting more energy towards you. But, if it doesn't matter, it could be you aren't experiencing increased signal, but lower noise - that something in your immediate vicinity is raising the noise floor in the freq of interest, and once it gets cold, that something quiets down for some reason.

Noise tends to be broadbanded. If it is noise affecting your reception, then you would probably benefit from using 50 ohm cable. Good impedance matches mean a more efficient transfer of energy, closer coupling to the antenna meaning more benefit from any gain it has, greater rejection by any Q it possesses, and less noise created in the rcvr front end by proper loading. A receiver only has to do two things: receive the frequency of interest, and reject all others. Reception is a function of gain. Rejection is a function bandwidth. Impedance affects bandwidth and thus rejection making a receiver more susceptible to noise. I would investigate using 50 ohm cable from one antenna to one receiver. If that helps, then you can work on making a multicoupler to connect all your receivers.

Today it's about 44 degrees and I did one last final attempt on receiving those weak signals. I bought a $20 Coax booster (15dB gain) and put it inline about a foot after my antenna just to test. Plugged it in to 12volt dc power and I am receiving those signals again.

So i'm pretty sure whatever is happening it's due to line loss and when it's cold there is less loss in the line....only thing i can think of.

Thanks for all the help!

You're welcome smitty. I disagree with your analysis that cold decreases the loss in your line, and actually proves mine. I think by placing the amp closer to the antenna you overcame noise created by an impedance mismatch with the antenna. Once cables are far shorter than a wavelength, their impedance is essentially zero. At about a foot long, slightly under a quarterwave, you may have accidentally found the perfect match since most 50 ohm antennas are actually around 38 ohms. So, you have a clean, quiet connection to the antenna, and by sheer brute force send enough gain down the line to overcome noise created in the front end of the radio. That's not to say you will receive these stations perfectly if you replaced the RG6 with RG58 or LMR400. You might still an amp to receive these stations reliably. But, I think it points out the effects impedance plays when you are at the performance limits of a receiver - that RG6 is adequate for local stations with good signal strength well above the noise floor of the rcvr, but masks the signals of far stations with noise created in the rcvr front end. YMMV of course.

It also points out the merits of using TTA's (tower top amplifiers) driving a long line to the receiver multicoupler. Properly sized and filtered for the system design, they can significantly improve the inbound performance of any repeater. That's a whole topic itself.

I still like your set up though. Neatly done, and obviously well assembled. You are on the right path. It think it's great. Enjoy!

I keep seeing people using RG6 in monitoring applications, and I think it comes back to an old article Bob Grove posted in Monitoring Times that got picked up by the Police Call books in their scanner newbie guide. Personally, I have never seen it work well; yes, on paper the impedance mismatch impact may be minimal but in the real world it's killed by two things: cheap cable and cheap cable TV splitters. If you absolutely have to use it, use Belden RG6 Quad Shield. Also look into getting a commercial cable head-end preamp/multicoupler combo that will allow you to adjust the individual gain on each port.

Personally, if I was putting together a 4 radio VHF-Hi monitoring setup with a 100' cable run, I'd use this antenna, LMR400 with N connectors for the feedline, and put Mini-UHF F-M-F T-connectors on the backs of the radios and daisy-chain them together with no more than a foot of cable (LMR195 or RG142 if you can get it) between each radio. Don't laugh, it works great, and the gain of the antenna is more than sufficient to overcome the feed line loss at 150 MHz.