Disappointing reception - antenna placement?

[KF7ZXT]

I've finally finished installation of a GPS-enabled Uniden 996XT. I used an external trunk lid mount to mount the antenna on my hood. The antenna is a Larsen MNO 150/450/800 MHz antenna: Larsen NMO150/450/800 Tri Band VHF, UHF, 800 MHz Motorola Style Antenna | TESSCO

I was hoping a grounded, tri-band antenna (this antenna was recommended to me on this forum) would result in significantly better reception that the stock, table top antenna that came with the unit that I used (indoors) while programming). Unfortunately, reception seems somewhat worse - specifically, communications are getting clipped mid-transmission, despite being very strong signals on a trunked system.

I wonder if I placed the antenna too close to the window pillar. Rather than describing placement, attached is a photo

So, my questions are:

1. Is this poor antenna placement, causing interference by the vehicle body?
2. Is there a test I can use to determine interference? (I don't have such instruments, but have friends who might)
3. Is this just a poor quality antenna? If so, could I screw a better antenna into the mount I've already fixed.

I opted not to go for a roof mount simply because I spend a fair amount of time in low-ceilinged garages and dont want to put an antenna through that kind of abuse.

Many thanks.

 

[jackj]

That location isn't ideal but it should work. You might get some ignition noise though.

Unscrew the antenna from the base and use a VOM (volt-ohm meter) to check the continuity between the center conductor of the connector that connects to your scanner and the center contact on the NMO base. You should show a short. Then check between that center conductor and the outer conductor, you should not show a short. If you don't already have one, you can get a VOM at most auto stores, hardware stores or home supply stores.

If your readings don't match then you need to find out where the problem is and correct it. Then your antenna should work fine. Good luck.

 

[KF7ZXT]

Jackj,

Thanks. I don't seem to be getting an ignition noise.

I do have a VOM (I'm not skilled at using it, but can probably figure it out. Anyway, I presume the scanner needs to be turned on for the test right? Silly question, I know.

Also, in this context, how would I detect a short?

Many thanks.

 

[LtDoc]

All that you can check with any type of multimeter is continuity. If it's working at all, I doubt if that's a concern, really. Considering the bands you are trying to cover, there isn't going to be any 'smallish' antenna that's going to give dramatic improvements. Some improvement on particular bands, sure. But nothing fantastic. I don't have that antenna so can't really make a comparison with it to anything. Sorry.
- 'Doc

 

[KF7ZXT]

LtDoc,

Could you make a suggestion on a taller antenna that would screw into the same mount?

Many thanks

 

[RadioDaze]

Do the mounting screws actually bite through the paint to make a true electrical ground connection? They need to contact the metal of the vehicle.

 

[KF7ZXT]

Radio daze,

I wasn't sure I was drilling into much more than polycarbonate, so I actually ran a ground loop to something that was definitely grounded.

 

[mmckenna]

A longer antenna might work better on VHF, maybe UHF, but not on 700 and 800MHz. Since you didn't tell us exactly what frequency is doing this, it's sort of difficult to make any recommendations.

As for a "very strong signal on a trunked system" cutting out, I'm wondering if it's working to well and you are overloading the front end of the scanner. Scanners have pretty broad front ends, and it's easy to overload them with strong nearby signals. What I'd try is when you get in a situation/location where this is happening, try swapping the stock antenna back on the scanner and see what happens.

As for testing the install with a multi-meter, you need to disconnect the coax from the radio. Remove the antenna from the mount.
Test for continuity between the center contact and the outer contact. There should be NO continuity between those. If there is, you need to find the location of that fault and fix it.
Test for continuity between the outer contact and a known good ground on your vehicle. Try the negative battery post, engine block, frame, etc. There should be continuity between those.
Test for continuity between the center pin of the antenna mount and the center pin of the coaxial connector on the end that would have been plugged into the scanner. You might need some help with this. There should be continuity between those two.
Last, test for continuity between the outside of the coaxial connector that would have been plugged into the scanner and a known good ground. There should be continuity between those.

If any of these test fail, find the fault and fix it. Areas that can have issues:
Any place where the cable can rub against sharp metal.
Any place where the cable can be pinched.
The connector at the end of the coax.
The antenna mount.

Lastly, make sure your scanner is getting nice clean power. Ideally you want the power feed for your scanner coming directly off the battery. Not the cigarette lighter, not taped into another fuse or wire, but a straight run to the battery posts with proper fuses.

Address all those issues, including the antenna swap test, and I think you'll find the issue.

 

[KF7ZXT]

Thank you for the very thorough answer. I will safe this for future reference.

I'm embarrassed to say I think I tracked down the problem, which is vastly less complicated. After comparing my table top (not mobile) 996XT to an old Radio Shack Pro-93 - I have - monitoring them both on a single trunked system, I was totally flummoxed why my cheap handheld was catching everything, while the 996 kept cutting out in the middle of transmissions with a strong signal.

I did a little more research. It appears the problem is that I had WX alert switched on, causing the scanner to cut-out every five seconds, and a couple more to recover. I turned WX alert off, and it seemed to solve the problem. Reprogrammed my digital system and presto - problem solved.

My antenna set-up may need to be evaluated at some point - I've only used it an area of very strong RF, but it seems the imminent problem is solved.

Thanks

 

[nanZor]

Unscrew the antenna from the base and use a VOM (volt-ohm meter) to check the continuity between the center conductor of the connector that connects to your scanner and the center contact on the NMO base. You should show a short.

Good advice. And although the ops problem has been solved, after checking the nmo base center conductor, I go further and make sure there is actual contact with the whip itself. If painted black, I'll expose a very tiny amount of bare metal and check for continuity again.

The reason for this is that sometimes an nmo can have a flat pressure-finger that doesn't make full contact with the mount center conductor. Too large of a rubber o-ring, or minor manufacturing variances can leave you with a capacitive , but not direct connection. You'll receive signals, perhaps somewhat poorer than expected. A transmitter will reveal this capacitive coupling in a variety of ways, but might go unnoticed in our rx-only application.

 

[jackj]

I'm glad you found your problem. I've done the same thing in the past and it took me several hours to realize what I'd done.

For future reference:
In order to check continuity, set your VOM to ohms and the range scale to X-1 or the lowest scale. Short the meter lead probes together and the meter should swing to full scale (or close to it). When you are testing for continuity, the meter should show close to full scale. After you're done with the meter, set the range back to OFF or to the highest voltage range. This will keep you from burning out the meter if you connect the test leads to a voltage source while it's in the Ohms setting.

 

[LtDoc]

Can I recommend a larger/longer antenna for your purpose? No, I can't. A 'larger'/longer antenna for 800 Mhz can be of several types. One of them is just a longer conductor, may help but don't count on it. Another type would be a 'vertical array', meaning several 'right sized' elements stacked on top of each other and 'phased'/timed so that a signal is increased by adding all those separate antenna's signal together. No, that's not an accurate way of describing what happens but it's generally what does happen.
I don't listen to 800 Mhz so haven't looked to see what's available for that band. Good luck.
- 'Doc

And it seems that you've found your 'problem'! It's always the 'little' thingys ain't it?

 

[N5TWB]

OP - you have the widely-used and highly-recommended Larsen NMO 150/450/800. Now that you've solved your WX scan issue (most of us have done that), you should allow some time to fully evaluate performance as you make your usual travels in the areas you've set up in the scanner. The only way that antenna does better is on the roof and I understand that yours can't go there. You could spend $80-100 to get an Austin Spectra that is longer and used/liked by many scannists on this forum but its length may still be a problem in the parking garage, even with the hood mount. Like many things in life, trade-offs must be made and this is just another on that list.

 

[mmckenna]

Good advice. And although the ops problem has been solved, after checking the nmo base center conductor, I go further and make sure there is actual contact with the whip itself. If painted black, I'll expose a very tiny amount of bare metal and check for continuity again.

The reason for this is that sometimes an nmo can have a flat pressure-finger that doesn't make full contact with the mount center conductor. Too large of a rubber o-ring, or minor manufacturing variances can leave you with a capacitive , but not direct connection. You'll receive signals, perhaps somewhat poorer than expected. A transmitter will reveal this capacitive coupling in a variety of ways, but might go unnoticed in our rx-only application.

Excellent point. I've never used one of the 150/450/800 antennas and while I was pretty sure they are a DC coupled antenna, I didn't want to assume. On some antennas, like the 5/8th's, one side of the coil can be grounded. Testing for a short from center pin to ground on the radio connector end can indicate a center conductor to ground short.

One thing I've started doing is about once a year I'll fully disassemble the antenna and clean everything. I'll pull the whip about and use a soft wire brush in the drill press to clean any corrosion off the bottom of the whip where the set screw goes. I have some needle files that I'll use to clean the inside of the cone. I'll pull the set screws out and clean them up. I'll use some armor-all on the seals to keep them from deteriorating, and I'll clean the NMO mount on the truck. It's also a good time to make sure everything is good and tight. I'll put it all back together and check the SWR to make sure everything is still working well.

All to often the antenna gets neglected. I've pulled antennas off of vehicles only to find a corroded mess underneath. Last one I did this on has cooked the finals on a 110watt MCS-2000. Not a cheap fix and could have been easily prevented.

 

[hill2jb]

Thank you for starting this thread. I had a few questions brewing in my mind that were answered here. I will be installing the same radio, and same antenna, in my truck this week or next week hopefully. Antenna is already here, and I'll try to take pics on that as i install tomorrow. Doing a roof mount.