Antenna problem or concern and question

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SCANMUNCIE

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Got a antenna question that I'm not sure if its the antenna or if while switching antennas i might have damaged the mount.

My antenna is a roof top mount NMO connector. Drilled thru the roof of the car.

I had the following antenna on the car (Antenex Quarterwave - Unity Gain 19")

QW144 - Antenex Quarterwave Antenna (144-152MHz, Unity Gain, Stainless)

i upgraded from a 2m radio to a dual band radio.

Here is where my problem comes into play.

I installed the new dual band radio. Used the weather radio to test reception. After installing radio i turned in on and tuned into the weather radio freq (162.550) which is located 60 miles from where i was, using the above antenna (Antenex)

Recieved 7 of 9 bars on S meter.

Turned off radio and then went to upgrade the antenna to a Diamond NR72BNMO (Dual Band Antenna)

Diamond® Antenna ~ NR72BNMO Dualband Mobile Antenna

After installing the Diamond antenna turned the radio back on the the weather radio freq (162.550) now only had 1 of 9 bars on s meter.

Took Diamond back off and put Antenex back on and had 6 of 9 bars. Took off and put Diamond back on still 1 of 9 bars on s meter. The vehicles position didn't not change during antenna switching.

Looking at the antennas should there be that much difference?

The Diamond has gain as to where the antenex is just a unity gain.

Is it possible to have damaged the mount during the antenna switch?

Any suggestions on what to check?
 

ka3jjz

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My first thought is to make sure the mount is actually grounded, as the antenna ad says that is required. A simple check with a VOM will suffice.

There isn't all that much gain on the Diamond. The other possibility here is that it may not be broadbanded enough to get up to 162 mhz with reasonable efficiency. That might be an issue, since it is base-loaded, and it is physically smaller than your quarter wave. I would question the efficiency of the antenna so far from the ham bands.

A simple way to test this is to borrow someone else's NMO dual band antenna that is physically larger than your quarter wave and redo the test. If the signal strength is comparable or better than the original antenna, you know that little Diamond is not very efficient outside its design parameters

73 Mike
 
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kb2vxa

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Don't bother going through all the changes when the answer is so easy even a caveman can do it. The difference is all in the antennas, in order to exhibit gain the dual bander is band specific so it will not receive out of band signals nearly as well as a quarter wave which has a MUCH broader "wide band" curve. In other words the "power" of the antenna is concentrated where it is most needed, out of band reception falls by the wayside.

There is nothing wrong with your antenna or the mount, we are controlling transmission. For the next hour we will control all that you hear and take you on a journey from the inner mind to... The Outer Limits.
 

DPD1

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I doubt anything is broken, unless the ground isn't connected. The dual band is a compromise, even though they claim the gain. Whereas, even though the other one was just a quarter wave, it still was dedicated VHF, and probably wide band enough to pull in the 162 better. It looks like the new one is VHF by combining both halves, then UHF on the one half. Combining two sections to create one 1/2 wave isn't as efficient as one true 1/2 wave element, though they probably want you to think it is. Something better would be bigger. But any dual band usually isn't going to be better than a dedicated one for each band.

I see Warren beat me to it.
 

RadioDaze

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The 6/9 vs. 7/9 is not a big difference, so the Antenex performance probably didn't really change much. I would take a little fine grit sandpaper, like 600 or finer, and gently sand the center contact on the NMO mount. If there's a touch of oxidation, maybe the Comet isn't making contact with the same pressure as the Antenex, and perhaps the Antenex is not making the same contact that it did before you first removed it. Having said that, the other boys are right, and the WX freqs should be considered "out of band" for that antenna. That antenna, which I've used before, presents a compromise for folks who want to go into a garage without having to get out and remove it. Still not a bad antenna, though, and I wish I could locate the box that mine must be hiding in.
 

prcguy

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Another vote for the Diamond is not tuned for 162MHz and thats the major reason for the reduced performance. Try comparing the two antennas on a 2m repeater and they should be very close.

The Diamond antenna has no gain to speak of, 2.15dBi is about the same as 0dBd or the same gain as a dipole. I would not sand your NMO contact without inspecting it very closely, some center contacts are silver plated and very thin.
prcguy
 

Luis_R

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Don't bother going through all the changes when the answer is so easy even a caveman can do it. The difference is all in the antennas, in order to exhibit gain the dual bander is band specific so it will not receive out of band signals nearly as well as a quarter wave which has a MUCH broader "wide band" curve. In other words the "power" of the antenna is concentrated where it is most needed, out of band reception falls by the wayside.
I second this!
 

SCANMUNCIE

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Thanks everyone for your reply's... I really didn't think about the freq as the 1/4 Antenex antenna was for the 2m band also so i just assumed the Diamond would receive as well despite receiving out of band.

One additional question is if i wanted to use a dedicated antenna for both bands what would i need to do that?
 
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