Whip antenna worn down from parking garages

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STEVE392

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I just noticed the whip antenna that I have on my ford F150 that is attached via nmo mount to my roof is worn down on the tip. The small ball that was there is longgone and the last 4 to 6 inches are worn at an angle from the parking garages in my area. I've had this antenna for like 6 years so it's had its fair share of abuse but will this worn down part affect any reception?
 

K4EET

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Hi Steve, could you tell us what the antenna is used for? Also, is it just the ball that is missing or is the whip perhaps missing an inch or two of metal? Finally, and the answer to the first question might nail this one, is this for receive only or do you transmit as well? Thanks in advance! Dave
 
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STEVE392

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Antenna is used to listen to railroad frequencies only, no transmitting at all. The antenna could very well be missing an inch or two. I haven't taken it off and measured. It's supposed to be 43" in length.
 

K4EET

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What frequency band are you listening in? I am not familiar with rail systems. Is that low VHF, VHF, etc.?

<edit> The reason I am asking is this. If you are missing an inch or two on the whip, depending on the frequency of operation will determine how much of a difference you might see in reception. </edit>
 
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K4EET

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Sorry about that, 160-161mhz vhf
OK then, it's in the VHF band. If you were transmitting, a couple of inches would change the SWR considerably. As for reception, can you tell the difference, I kind of doubt it at this point. Definitely not optimal but maybe not bad enough yet to warrant a new whip. If I were you, I would take the antenna off and measure the whip to see how much is actually missing. There must be a coil at the base probably making this a 5/8th wave antenna. But that is just a guess on my part without more information.

So Steve, if you get more information or want to post a picture or even identify the antenna by manufacturer and model number, I may be able to give you a more definitive answer.

Cheers! Dave
 
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a417

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As you said it's not "transmitting at all", you will probably not notice much of a difference. At that length and receive only I don't think you have much to worry about.
 

NC1

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I just ran the math, and 43" is nowhere near the rail frequencies. A half wave on 160.5MHz works out to 35" (it's actually 34.990" but I'm rounding up 1/100th of an inch).
If it were a 1/4 wave antenna, you would be at 17.5" (actually 17.495", but again I'm rounding up). So this leaves me a bit confused because the 43" does not even come close. If it were me, I would cut it down to 35" so the antenna would be resonant.
 
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I just ran the math, and 43" is nowhere near the rail frequencies. A half wave on 160.5MHz works out to 35" (it's actually 34.990" but I'm rounding up 1/100th of an inch).
If it were a 1/4 wave antenna, you would be at 17.5" (actually 17.495", but again I'm rounding up). So this leaves me a bit confused because the 43" does not even come close. If it were me, I would cut it down to 35" so the antenna would be resonant.

The math is good for a dipole but this is a mobile antenna with a load or two I'll bet.. Or, like me sometimes, dyslexia sets in and his antenna is actually 34" tall???? Ah but a model number or good picture may lead to what this mysteriously loong NMO mounted antenna is...???
 

K4EET

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Unless this antenna is completely wrong from the get-go, I would say it is base loaded. Unless you know the exact particulars of the antenna, one cannot determine how long the whip should be. Whatever Steve does right now...

Do NOT cut anything!!!

We do not have enough information to determine how long the whip is supposed to be.
 

a417

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It's already shorter than it used to be, as he described repetitive friction damage to the distal end...If he's not transmitting,and he's not having issues with reception - he very well could leave well enough alone.
 

NC1

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The math is good for a dipole but this is a mobile antenna with a load or two I'll bet.. Or, like me sometimes, dyslexia sets in and his antenna is actually 34" tall???? Ah but a model number or good picture may lead to what this mysteriously loong NMO mounted antenna is...???

If it were base loaded, then that changes everything for sure.
A 5/8 is in the ballpark of 43", but that's for transmit to get the signal out further, and would not be optimal for receive.
When we know what it actually is, I'll have to get my ARRL Antenna handbook off the shelf.
 

prcguy

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A 5/8 wave NMO is very typical for RR only monitoring and it would be in the 43" range. 5/8 is not just for transmitting, all antennas are reciprocal and whatever advantage it has on transmit it will have on receive. If I did RR monitoring from my vehicle I would use a 5/8 wave Larsen.

If it were base loaded, then that changes everything for sure.
A 5/8 is in the ballpark of 43", but that's for transmit to get the signal out further, and would not be optimal for receive.
When we know what it actually is, I'll have to get my ARRL Antenna handbook off the shelf.
 

jhooten

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43" is correct for an NMO-150.

36410378.jpg
 

STEVE392

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This is hte exact antenna I have but it is no longer mag mount. The mag mount rotted from rust so I bought a hard mount and drilled it into my roof.
 

NC1

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A 5/8 is still not the best for a mobile installation. Let's look into that a little bit.
Just like statistics and our government reports, they can lie. Notice that the well circulated 5/8 vs 1/4 modeling represents perfect ground. A vehicle is not a perfect ground extending out tens+ wavelengths. For explanation purposes I'll use the amateur 2m band. Perhaps if one places a 2 meter 5/8 wave antenna on a lake freighter we would see a 5/8 wave advantage bow to stern. Model the 5/8 wave over a ground plane typical of the average sedan one would see the 1/4 wave clearly outperform the high angle radiation of a 5/8 radiator over the minuscule and imperfect ground. Side to side, a typical car's ground image is barely over a 1/4 wavelength; which is very far from perfect ground. Ground loss will affect the pattern, especially the gain at low angles such as the 5/8 antenna.

If one needs more gain on the horizontal plane, look at purchasing a collinear stack. Bottom line is that the 5/8 wavelength antennas are not the best performers in a mobile instillation and 1/4 wins in a typical mobile instillation with typical varying conditions as a mobile would experience. The theoretical difference is about 1.2 dB, but in the real world where terrain always varies and will be hilly or even mountainous, the 1/4-wave is the better performer. I know I'll get the anecdotal evidence crowd to defend their installation, and the ones who have been designing antennas professionally for 120 years with antenna theory and EZNEC modeling, but the 5/8 is really for base installs that can actually take full advantage of being a 5/8.

As for me, I use 1/4 wave on all vehicles mainly because I live in hilly terrain and they just work better. Not to mention I can pull into my garage without hitting anything.
 

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jhooten

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NC1,
If the major brand manufacturers of quality antennas only depended on modeling you may have a point. They don't. In addition to modeling (back to this one shortly) they mount their products on real vehicles which spend many hours on test ranges with calibrated field strength meters plotting the actual patterns.

Next on to modeling. Hobby grade antenna modeling programs may only give you the option of perfect ground. On one of my trips to Schaumberg an engineer showed us the program they use. He was able to select wire frame models of a large number of vehicles, from compact cars to heavy construction equipment, and move the antenna around on them. They guys who design and test antennas for a living may be smarter than you give them credit.
 
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