At 941MHz how much will will putting antenna in attic affect reception?

[JoshuaHufford]

I've been trying to pull in a faint signal that is at 941 MHz without a lot of success. I decided to try a Yagi antenna, and setup a temporary setup today and reception was greatly improved and made it very usable. I'm thinking about trying it in my attic before I go to the trouble of mounting the antenna outside on a mast, grounding it, etc, the attic install would be quite a bit easier. The antenna was right at the level of the base of my roof so I should be able to get a few more feet in elevation, but it will be behind 2 layers of shingles and the plywood. Just curious if I should even bother getting up in the attic to give it a try, won't be the easiest thing to do but I can do it. Here is a picture of the temp setup from today.

 

[iMONITOR]

Unless you have foil backed insulation and a lot of electrical wiring above the floor level of the attic it should work fine. Maybe you could try it out by clamping it to a wood beam or sit it on a cardboard/wood box in the attic. Temporarly run the coax down through the attic access and see how it works.

 

[RFI-EMI-GUY]

What is to hear on 941 MHz?

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[IcomIcR20]

If I take my HT into my attic, 800 MHz signals go up several units on the signal meter. I've been able to successfully pull in P25 from 70 mi away.

I'd give it a go!

 

[trentbob]

At 900 megahertz the biggest problem you're going to have with that antenna in the Attic will be if you use any coax that isn't high quality low loss coax.

 

[JoshuaHufford]

It is an ATCS signal for railroad communication. It will allow me to see when a train is lined/authorized to come into town from the dispatcher.

The coax run should be pretty short, 6-7 Ft. at most. I will be able to put the radio right below the attic access on the top stair. The only time I will need to move it will be when I need to get in the attic which is not very often.

Thanks everyone, sounds like it will be worth giving it a try.

 

[trentbob]

Yep, at 900 megahertz even a short distance it is worth buying 10 ft of lmr400 or RG6. Even a short distance will make a difference.

 

[RFI-EMI-GUY]

For receive only, RG6U has low loss.

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[Ubbe]

The antenna are amplified, you can see the bias-T power inserter at the receiver, so no worries about the coax quality, use cheap RG6 and as long as you need. As iMonitor mentions it could be the insulation and moist protection that sometimes are plastic foil with a metal component that could attenuate high frequency signals. But that are usually used in modern houses and you might not have that in the attic. If that train signal are from the dispatcher it will always be the same strenght and it is not from the trains transmitter? Then it should be easy to compare signal strenghts from outside and when you hold the antenna in you hands in the attic.

/Ubbe

 

[Frankhappyg]

I would mount it on the roof myself.
Even a 3-4 foot mast mounted to a vent stack would be sufficient and greatly increase your range and potential to pick up other distant systems.



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[JoshuaHufford]

Actually that is the preamp right before the receiver. The signal was present without the preamp but I could hardly decode anything, when I added the preamp things were decoding very well. There is 6 feet of LMR-400 between the antenna connection and preamp.

The house was built in the late 60s so I doubt there will be anything metal in the roof, and all of the wiring will be below the antenna.

The signal is from a base station and will not change.

As far as picking up other systems, there are stations (control points) along the tracks that I would love to be able to pick up, but they are at lower elevation, and they also use directional antennas pointed almost exactly away from my location, pretty sure they are lower wattage as well. They are also on a different frequency so that would require another radio, power divider, etc. complicating the setup. I might try it just to see but I'm not hopeful. The next base station down the tracks is twice the distance this one is, and is probably 20-30 degrees to the west. I already know an Omni antenna will not work, and this one is pretty faint, so it looks as if this transmission is the only one I will be able to receive no matter what, unless I install a huge tower which isn't an option. So if I can pick up this one signal in the attic it will be pretty much all I can get anyway and I don't see the need to go through the extra work of putting it outside. The vent stacks on this house are small and only about 1 ft. above the roof, probably not up to today's codes at all, I don't know if installing a 5 ft. yagi on that small of a vent stack is a good idea. I'm going to try it in the attic with a temp setup for a week or two just to see how reception is, just want to make sure there wasn't skip going on when I was testing yesterday. If the attic location ends up not working then I'll look into a location outside.

Thanks, this has been very helpful.

 

[K8EAA]

RG6 is 75 ohm. Could be some loss on a mismatch. Maybe not enough to worry about.

 

[dlwtrunked]

RG6 is 75 ohm. Could be some loss on a mismatch. Maybe not enough to worry about.

With good cable, the loss is about 2dB from the impedance mismatch. This is not detectable in receiving except by careful measurement. The quality of each of the cables being compared and their attenuation at the desired frequency is going to be a greater factor. The type and height of the antenna are also far greater factors. (Do not try an broadband impedance transformer as it will just add more loss.) I use both RG-6 quad shield and LMR-400. At 800 MHz, the LRM-400 does better but that is mostly not due to the impedance but rather it has lower loss (about 4 dB less for 100 ft) at 800 MHz.

 

[iMONITOR]

RG6 is 75 ohm. Could be some loss on a mismatch. Maybe not enough to worry about.

Bob Grove from Monitoring Times Magazine used to test, discuss and recommend RG6, as did Bob Parnass, to the point of exhaustion! They insisted the impedance difference with 75 ohm instead of 50 ohms was never an issue for receive only applications, especially within our scanner hobby and the frequencies they operated at. For those who haven't had the pleasure of knowing them, they were both iconic in our hobby, very knowledgeable and helpful for decades!

 

[nd5y]

What is to hear on 941 MHz?

In case anybody is curious.
eCFR — Code of Federal Regulations
ULS License - Microwave Industrial/Business Pool License - WQAT628 - UNION PACIFIC RAILROAD COMPANY

 

[Ubbe]

With good cable, the loss is about 2dB from the impedance mismatch.

You forgot the decimal, it's 0.2dB difference if both ends of a 75ohm coax are 50 ohm, but they seldom are.
75ohm are more effecient as a recieve coax with less loss than a 50ohm coax if their matching are ok and are the reason why it is used in every receive installation of cable tv, satellite, and broadscast signals in buildings.

/Ubbe

 

[trentbob]

Bob Grove from Monitoring Times Magazine used to test, discuss and recommend RG6, as did Bob Parnass, to the point of exhaustion! They insisted the impedance difference with 75 ohm instead of 50 ohms was never an issue for receive only applications, especially within our scanner hobby and the frequencies they operated at. For those who haven't had the pleasure of knowing them, they were both iconic in our hobby, very knowledgeable and helpful for decades!

As was Tom Kneitel and Gene Hughes, I published a frequency guide around the same time Tom owned crb research in Commack New York long before there was an internet but we are going way back here.

I only recommended the lmr400 or RG6 because of the 900 megahertz frequency. I assumed we were listening only here and even at 10 feet the coax is important or you will lose reception. As far as impedance difference it doesn't matter as Bob Grove would say but as long as you're going to the trouble to put the antenna in the Attic you might as well use a low loss coax for less signal loss. This has gotten way too complicated LOL.

 

[JoshuaHufford]

Well I tried it out today and guess what, it didn't work. I could still receive the signal but it was too weak to decode anything. I took it back outside and it was much better, not quite as good as it yesterday but still much better.

So it looks like outside it will be! I have a few ideas on placement but I'll start a new thread for that.

 

[trentbob]

I don't think you have any problem continuing on with this thread. I'm curious to see how it goes. Your application is so specific and the antenna is so maneuverable you will figure it out. Just keep with it and let us know here. Good luck brother.

 

[KMG54]

I know people love their high end coax here, but good quality RG6 is swept to 3 Gigs, nothing wrong with using it for receive. In fact it is better than a lot of the 50 ohm stuff sold on the cheap. Run it and use it and enjoy the frequencies.