Improve 800 mHz on Larsen tri-band

[rgvscnr]

Are there any cell (nextel) towers in the area where you're seeing the signal drop? You could have some overload issues going on. This could explain why adding a better antenna has resulted in worse reception. you could try adding attenuation, and see what happens.

just a thought
jeff.

Jeff,

Yes! Quite a few cell towers. I live near the US/MX border and for some reason there are cell antennas everywhere and now that I read your post I can correlate reception quality to the proximity to these antennas. Don’t know how many of them are Nextel though.

I am still going to try the different antenna and the High Freq. cable as suggested above just to see what happens.

Thanks.

 

[dougr1252]

Larsen seems to specify dual-shield cable for all their mobile stuff, but the NMOHF mount is definitely much cleaner than a normal NMO at 800 MHz if you sweep it. The tri-band antenna is an excellent performer on all three bands.

http://www.larsen-antennas.com/docfiles/ASB9/SalesSheets/NMOHFMounts.pdf

Sometimes the best setup is an external antenna _and_ attenuation turned on in the scanner. The antenna brings in more of the desired signals, and the attenuator cuts down on the overload and images / intermod / garbage that you get on a scanner. Scanner with plastic cases are also vulnerable to multipath when 800 MHz signals go through the case directly onto the circuit board, and also arrive slightly later via the coax. See if you can still hear signals with no antenna.

It's probably not the antenna, but the mount (more specifically the coax used on the mount). The Larson NMO mounts come in various types. The most common use standard RG-58 coax (the label this CX type coax) and is not recommended for frequencies above 512 MHz. They also sell a "high frequency" NMO mount that uses a type of RG-58 (they call it DS type coax) for 800 MHz band installations.

 

[Kumba]

This is exactly the forum thread I needed....

Was looking for ways to increase the range on my car's mobile antenna to try and pick up my home county as far away as possible. I'm in the DC Area, and on a clear night, I pick up my home county (Charles in MD) up by the roads near the Pentagon, but only for brief spurts. I don't know what they transmit at, but they have to have one of the strongest signals in the area for it to get that far. I can't pick up Calvert or St. Mary's county at all while at home, and they're next door to Charles.

So I was initially looking at getting one of those stub antennas to compliment the Larsen tri-band I already have, and just use a switch box to change between the two antennas as needed. But reading this thread here suggests I may need to just get a different NMO Mount? I think I have the standard Larsen NMO that's got 17ft of thin (RG-58?) coax, so it sounds like that might be something to change out first. I'm guessing RG-6 with a smaller cable run? But would it be wise to buy RG-6 myself and just re-wire the NMO mount, or just get a newer NMO mount?

For reference, this was the low-profile antenna I was looking at:
TRAB8213 - Antenex Phantom Low Profile 821-896MHz 3dB-MEG Gain Antenna (Black/NMO Mount) with Patented Field Diversity

Edit: Forgot to add, my NMO mount is the Larsen trunk lip version.

 

[fineshot1]

This is exactly the forum thread I needed....

Was looking for ways to increase the range on my car's mobile antenna to try and pick up my home county as far away as possible. I'm in the DC Area, and on a clear night, I pick up my home county (Charles in MD) up by the roads near the Pentagon, but only for brief spurts. I don't know what they transmit at, but they have to have one of the strongest signals in the area for it to get that far. I can't pick up Calvert or St. Mary's county at all while at home, and they're next door to Charles.

So I was initially looking at getting one of those stub antennas to compliment the Larsen tri-band I already have, and just use a switch box to change between the two antennas as needed. But reading this thread here suggests I may need to just get a different NMO Mount? I think I have the standard Larsen NMO that's got 17ft of thin (RG-58?) coax, so it sounds like that might be something to change out first. I'm guessing RG-6 with a smaller cable run? But would it be wise to buy RG-6 myself and just re-wire the NMO mount, or just get a newer NMO mount?

For reference, this was the low-profile antenna I was looking at:
TRAB8213 - Antenex Phantom Low Profile 821-896MHz 3dB-MEG Gain Antenna (Black/NMO Mount) with Patented Field Diversity

Edit: Forgot to add, my NMO mount is the Larsen trunk lip version.

You seem to under the wrong impression about your antenna needs.

A low profile antenna (like the one you are referring to) is a compramise antenna
and will not increase your reception on 800Mhz for distant stations, in fact it will
have the opposite effect. Low profile antennas are meant to be used in a high
signal area environment where there is plenty of signal to capture. If the Larsen
is not working for you on 800 and you wish to try another, going to a low profile
antenna is not the way and it sounds as if you are trying to receive signals from
a mobile that may not be receivable while being mobile which you should consider.

Also, changing your antenna mount to one with better 800 Mhz receive performance may
help but it will not likely be a "fix" to your problem.

 

[Kumba]

You seem to under the wrong impression about your antenna needs.

Pretty much hits the nail on the head here. I'm more of a computer/IT guy, so the radio tech is still very new to me, but I dabble when I can.

I should note that my current setup works very well, actually. I'm just looking for ways to maybe increase the range of receiving the 800MHz (868.*MHz, actually) that my home county's public safety system operates on. Since I know that I can sometimes pick them up on the southern side of DC if the conditions are right, I figure tweaking my current setup might work and I'll just keep the one antenna. But I wasn't sure if a dedicated 800MHz antenna would work better, using an antenna switch to change between them (i.e., Diamond CX-210).

So reading here, it seems cable length and such was something that might be worth tackling, but I wanted to get some clarification before jumping at it. The current NMO has a 17ft run, so shortening that will at least cut back on the amount of wire in the backseat of my car ('07 Camry). That, and changing to the thicker coax, but I'm guessing I might have to do that myself. (which doesn't look too hard).

 

[fineshot1]

But I wasn't sure if a dedicated 800MHz antenna would work better, using an antenna switch to change between them (i.e., Diamond CX-210).

So reading here, it seems cable length and such was something that might be worth tackling, but I wanted to get some clarification before jumping at it. The current NMO has a 17ft run, so shortening that will at least cut back on the amount of wire in the backseat of my car ('07 Camry). That, and changing to the thicker coax, but I'm guessing I might have to do that myself. (which doesn't look too hard).

The idea of a dedicated 800 antenna is sound but your choice was poor and that's all I was pointing out.
The cable shortening idea is also a sound idea. Thicker coax (depending on what type) may be impractical
in a mobile environment. I have used LMR240 in the mobile and the improvement was noticeable but care
must be taken installing it as it is much more rigid than regular cable. I made my own nmo mount with it
and it works very well. The soldering was touchy and it took me several attempts before I got it right.

 

[Kumba]

The idea of a dedicated 800 antenna is sound but your choice was poor and that's all I was pointing out.

Thanks! I'm quite unfamiliar with Antenna-speak, but learning.

The cable shortening idea is also a sound idea. Thicker coax (depending on what type) may be impractical in a mobile environment. I have used LMR240 in the mobile and the improvement was noticeable but care must be taken installing it as it is much more rigid than regular cable.

This shouldn't be too much of a factor...Camry's have smooth curves all over the place that a cable can follow, but I'm more inclined to thread it through the crevice in the back seat and straight up the center console (a.k.a., poor man's approach). I'm the only user of my car anyways, so looks don't matter much.

A quick search, however, turns up an LMR-240 variant using stranded copper instead of a solid-core, giving it more flexibility: LMR240 Flex Coaxial Cable

Thoughts on that?

I made my own nmo mount with it and it works very well. The soldering was touchy and it took me several attempts before I got it right.

Yeah, soldering is always a pain. I find I usually lack a powerful/hot-enough unit. I manage to get by, though. I'll eventually get a butane-powered one that should trump the electric models (at least the low-end ones).