Improve 800 mHz on Larsen tri-band

[rgvscnr]

Hello,

I just completed a Larsen tri-band mobile installation similar to this one: http://forums.radioreference.com/pictures-your-shack-mobile-setup/98560-05-toyota-tacoma.html in my 2008 Tacoma.

Reception on the 100s and 400s improved amazingly, but the 800s is just as bad as it was before with my Pro-95 rubber antenna.

Is there way to fine tune this antenna? I have freqs in all 3 bands, but I would rather optimize it for the 800s.

 

[KT4HX]

I will say I am surprised by that. I have always found the Larsen 150/450/800 to be better on 800 than the other two. Don't get me wrong, it works well on the others, but I have never had an issue with 800 MHz reception on this thing.

Hello,

I just completed a Larsen tri-band mobile installation similar to this one: http://forums.radioreference.com/pictures-your-shack-mobile-setup/98560-05-toyota-tacoma.html in my 2008 Tacoma.

Reception on the 100s and 400s improved amazingly, but the 800s is just as bad as it was before with my Pro-95 rubber antenna.

Is there way to fine tune this antenna? I have freqs in all 3 bands, but I would rather optimize it for the 800s.

 

[n5ims]

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.

 

[KT4HX]

I will say that I have never had any issues with 800 reception using a standard NMO mount using 10 ft of RG-58. That said, I am not aware of the length of cable or type that the OP is using.

 

[rgvscnr]

Here is some update info. The cable assembly was "left over" from a Motorola antenna part number HAF4013A. The coax with this assembly is RG58/U and I used about 7ft. The cable was terminated with a Rad Shack Right-Angle BNC connector (278-0126).

Reception is very strange. On a scale of 1-10, it is about a 6 when I am within 3 miles of the base antenna. After that it goes down to a 4 when I'm within approximately 12 miles and then it goes up to a 7 or 8 after that. And as I mentioned before, for the most part reception is better with the rubber antenna.

Any suggestions? How about that high frequency cable assembly?
Could it be a ground problem?

 

[3mary2]

Could it be the cable? I have never had any trouble with my Larsen and I have moved it at least four times. Is the connection at the mount good and clean on both ends. Dirt or rust will make for poor reception. Another thought purchase new cable and connector for the antenna and see if that corrects the problem. I have never known of any tuning for this antenna.

 

[radiopro52]

I'd check the cable connections and the mount. I have the Larsen Tri band with the Larsen NMO mount and it works very well in 800 MHz. In fact, reception in 800 MHz is comparable to my outdoor LP antenna.

 

[MOGA]

If the coax is a recycled part from another antenna base, then there's a good chance that assembling a new one, using quality parts, would be an improvement. In fact, I'm thinking of replacing the 58A/U in my larsen mobile scanning rig with some lower attenuation coax. I primarily scan P25 800 TRS and can use all the help I can get in getting a strong control channel to decode the traffic. Those decibels lost in using thin coax could come in handy. I'm stunned that I overlooked it until opening this thread, honestly.

 

[KT4HX]

I fully understand the loss factors associated with various cables, but sometimes we (and I've done this as well) get too wrapped up in the cable to cable loss comparison issue when it comes to short runs. As I mentioned previously, I have never had an issue using a mobile mount with the stock 10 ft section of RG-58 for 800 MHz reception. The loss of a 10 ft section of RG-58 at 850 MHz is 1.613 db. Of course something like LMR-400 would have much lower loss (.377 db). But to your ear, I really doubt you would notice that much difference unless you were in a fringe area where reception were spotty. Besides, RG-58 is much easier to work with in a mobile setting than something like LMR-400 (or even RG-6). That being said, I know some have opted to upgrade the coax for their mobile operations. It comes down to what makes the individual happy.

If the coax is a recycled part from another antenna base, then there's a good chance that assembling a new one, using quality parts, would be an improvement. In fact, I'm thinking of replacing the 58A/U in my larsen mobile scanning rig with some lower attenuation coax. I primarily scan P25 800 TRS and can use all the help I can get in getting a strong control channel to decode the traffic. Those decibels lost in using thin coax could come in handy. I'm stunned that I overlooked it until opening this thread, honestly.

 

[rankin39]

I've tested my Larsen triband against a number of NMO mount mobile antennas including a cellular (dedicated 800 MHz. 3 db.antenna) and the triband is equal to or better than any of them. I was pleasantly surprised. If you have trouble with 800, I'd say to get one of the 21 inch dedicated 800 antennas with the double coils. It will do a satisfactory job on the lower bands and will maximize your 800 performance.

Bob, WoNXN

 

[MOGA]

1) I'm in a location where reception is spotty

2) I have a dedicated 800 antenna, double coil (Larsen)

3) I have 17' lead, so the loss is likely considerable at 850 MHz.

I've some extra coax around that I need to utilize too. That's also a factor in deciding to re-wire the base.

 

[KT4HX]

1) Then you might benefit.

2) If your ant has a gain factor, then that will offset some of your loss.

3) The loss for 17 ft at 850 is 2.742 db (nearly half your signal).

4) I don't see that you "need" to utilize it, but since it's already paid for and can be used, that's good. If you don't mind my asking, what type and do you need 17 ft for your run? Also, since it will be bigger diameter, how are you going to feed it into the vehicle? Additionally, are you using a magmount or a drilled install? Using a magmount can also lessen your signal quality, and often times result in increased noise levels due to ineffective grounding.

1) I'm in a location where reception is spotty

2) I have a dedicated 800 antenna, double coil (Larsen)

3) I have 17' lead, so the loss is likely considerable at 850 MHz.

I've some extra coax around that I need to utilize too. That's also a factor in deciding to re-wire the base.

 

[MOGA]

Thanks for the kind words. I enjoy concerning myself about every little dB that I can squeeze out of a setup. It's just in my makeup to overbuild things, plus it's part of the hunt for the optimal implementation. Do it right the first time and you won't have to do it again, am I wrong? I could use that mag mount later on on a temporary basis while in my wife's vehicle. Otherwise I'll probably not worry about it as I've got a 3/4" antenna saw on the way. Going to make some holes in the roof of my SUV shortly. The New Motorola mount is a Maxrad; the current mag mount is Larsen. 17' is how long the lead is on the latter, not that I need that much, nor would I attach that length of replacement coax. But now that I think of it, speaking of ground loops from using mag mounts, am I losing anything while using the mag mount for 800 operation, considering that line loss is primarily a byproduct of TX? Still, part of what I like about radios is trying to capture as much signal as possible, so I'll probably still swap it out. OTOH, I've got bigger fish to fry, like looking for a fuse holder that will accommodate AWG 6 wire for power from the battery posts for the FT-7800 that's not going to cost me $60, etc. The more you put into radio operation, the more you realize you need. This pursuit is more expensive than guns, I tell you.

In case your curious, 6 AWG to provide 9A continuous at less than 0.25v drop over 15' run from the radio body to the battery terminals. I'm sure you know what the loss of a volt does to the output power of your FM transceiver. You certainly won't be dealing with 50w on 2m, unless you have the proper voltage getting to the radio, that's for sure.

 

[KT4HX]

Actually, coax loss isn't the exclusive domain of transmitting. Coax losses work both ways, so for every 3 db of loss, you half the signal that reaches the receiver. So if you had a coax run that has 3 db loss, and an antenna that has 3 db gain, then you effectively have a unity gain installation. Pretty much everyone that I know also has found a big difference when going from magmount to a drilled install - even on receive alone. One thing that I am sure you are aware of, when you install that 7800, be sure to bond all sections of the vehicle together to give yourself the best possible ground. Just in case you haven't seen this site, I highly recommend it: Welcome to KØBG's Web Site It has a lot of good information for mobile installations. By all means, taking the time to plan out an install for optimal performance is the right way to go. As the old saying goes, if anything's worth doing, it's worth doing right.

 

[rgvscnr]

Would the antenna be the Larsen NMO800? Larsen NMO800 [NMO800] - $29.95 : The Antenna Farm :: , Your Two Way Radio Source!

 

[KT4HX]

Not wishing to put words into MOGA's mouth, I would venture to say that is it, but he would have to verify that.

Also, my apologies if we kinda hijacked your thread. Hopefully you have picked up some info from our exchanges.

Would the antenna be the Larsen NMO800? Larsen NMO800 [NMO800] - $29.95 : The Antenna Farm :: , Your Two Way Radio Source!

 

[2531]

Mobile Antenna Comparison
Mobile Antenna Comparison - Intercept

 

[blueangel-eric]

Would the antenna be the Larsen NMO800? Larsen NMO800 [NMO800] - $29.95 : The Antenna Farm :: , Your Two Way Radio Source!

That is one coil with a spring load it looks like. i think this is like what he's talking about. except larsens isn't on the site. Antenex AB8065CT [AB8065CT] - $36.95 : The Antenna Farm :: , Your Two Way Radio Source!

 

[MOGA]

That is one coil with a spring load it looks like. i think this is like what he's talking about. except larsens isn't on the site. Antenex AB8065CT [AB8065CT] - $36.95 : The Antenna Farm :: , Your Two Way Radio Source!

It's just like this one, but sold with the Larsen trademark.

Larsen NMO5E825B High Gain NMO Antenna Enclosed Coil Whip

And I've pretty much given up on the 6 AWG power lines. It's just too much of a hassle finding power poles and fuses that have leads of that gauge. Heck, It's somewhat of a hassle to find even ring terminators that accept wire of that diameter. 15' of 8 AWG results in less than 0.2 v lost at 9A/14V and it's a heck of a lot cheaper to assemble the kit than with 6. Who would think that so little difference in wire diameter would result in a considerable difference in assembly cost? $85 v $25 for 25' of zip power cord, for instance.

OP, what have you decided, if anything? Are you going to replace the lead-in wire to see if that resolves the intermittent problems with reception? I know that's kind of a sucky proposition after you've completed your mobile install. I'm also curious if you experience better reception with another scanner/antenna arrangement? Have you otherwise ruled out distance from the transmitter as being the prime explanation for shoddy reception?

 

[JStemann]

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.