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Looking for antenna

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kjl13

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#1
Hello everyone from the Radio Refrence forums,

I am looking for a long nmo mount whip antenna(similar to the one below) that is around 100 inches tall(taller the better). The frequencies I need to use on it are 453.975, 453.4562, and 458.975, 458.4625. If anyone has any suggestions for something that long, let me know.
 

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W8RMH

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#2
Longer is not better. Antenna length depends on the frequency. The longer the antenna the lower the frequency. The antenna pictured is a low band antenna, 30 - 50 MHz. You could trim one of these down to UHF length (around 23 inches) but it would not perform well.

Something like the antenna below at 33 inches will work the best:

Larsen NMO450C
 
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#3
Hello everyone from the Radio Refrence forums,

I am looking for a long nmo mount whip antenna(similar to the one below) that is around 100 inches tall(taller the better).
Not sure if you are aware, but the antenna in the photo is not an NMO antenna.

I've never seen an NMO antenna that long. On the Larsen brand antenna, the longest whip I've seen them sell is 49 inches long.

The frequencies I need to use on it are 453.975, 453.4562, and 458.975, 458.4625. If anyone has any suggestions for something that long, let me know.
For receiving or transmitting?
If you are going to transmit, an antenna that long won't be resonate on those frequencies. If you really wanted a mobile UHF antenna that long (and I've never seen one for sale) it would need to be a custom built colinear design with a number of coils along its length.

For receiving, it's not going to work as well as a properly designed UHF antenna mounted on top of the vehicle.


Is there a reason why you are looking for an antenna that long?

If it's just the "look", then buy a low band antenna or CB whip and don't hook it up to anything.
 

kjl13

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#5
Not sure if you are aware, but the antenna in the photo is not an NMO antenna.

I've never seen an NMO antenna that long. On the Larsen brand antenna, the longest whip I've seen them sell is 49 inches long.



For receiving or transmitting?
If you are going to transmit, an antenna that long won't be resonate on those frequencies. If you really wanted a mobile UHF antenna that long (and I've never seen one for sale) it would need to be a custom built colinear design with a number of coils along its length.

For receiving, it's not going to work as well as a properly designed UHF antenna mounted on top of the vehicle.


Is there a reason why you are looking for an antenna that long?

If it's just the "look", then buy a low band antenna or CB whip and don't hook it up to anything.
I understand that it is not a nmo mount in the picture.
Secondly, it is not just for looks; thanks for the info.

EDIT:
I was looking for an antenna this long because I thought it would increase the range; but I guess not.
 
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The frequeny that is used with the pictured antenna is actually 853.6375.
The antenna you have circled in the photo is a low band VHF antenna. It's likely used for something between 30 and 50MHz.

The antenna in the photo that would be running 853.6375MHz would be one of the antennas on the roof, probably the low profile antenna just in front of the AM/FM whip.
 
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kjl13

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The antenna you have circled in the photo is a low band VHF antenna. It's likely used for something between 30 and 50MHz.

The antenna in the photo that would be running 853.6375MHz would be one of the antennas on the roof, probably the low profile antenna just in front of the AM/FM whip.
Why does the length of the antenna matter so much?
 

mancow

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#12
Why does the length of the antenna matter so much?
He was wrong, or he was referring to the black antenna on top.

Antennas are tuned to frequency. It's the same reason guitar strings are longer and shorter than others, to resonate on different frequencies.
 
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I spoke to a state police officer, just telling you what I was told. He may have been wrong, thanks for informing me though.
This is understandable, and common.
They are radio users, not radio techs. All they know is you push a button and talk, let it go and they talk back.
Most public safety officials get no more than a few minutes training on two way radio. There is rarely anything that touches on the technical side of it.

Would be nice if it was different, but your average EMT, fire fighter, or police officer has way more important stuff to do than learn the technology behind the radio systems.
 
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Why does the length of the antenna matter so much?
Yep, as Mancow said.
Antenna length is related to what frequency they resonate at.

In general terms:
A resonate antenna will transfer more energy into the air.
A non-resonate antenna will not work as efficiently and some of the energy will not get radiated, but instead reflected back towards the radio.

Higher radio frequencies have shorter wavelengths. Lower radio frequencies have longer wavelengths.

That's why a CB antenna (26 to 27MHz) is longer than a FRS radio antenna (462MHz & 467MHz)
 
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#15
Fundamental Frequency

Now what I'm going to say is basic stuff that everyone else here already knows, but maybe this will help the OP:

To figure out what the quarter wavelength of an antenna should be to be resonant for a particular frequency, you divide 234 by the frequency to get the length in feet. So here's the quarter wavelength for 853 Mhz:

234
----- = .275 or about 3"
853

Now let's say the quarter wave whip in your picture is for 40 Mhz. So the calculation would be:

234
----- = 5.85 or about 5' 10"
40

Perhaps this may help you understand why a 70" whip is not going to be resonant or effective at 853 Mhz.
 
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kjl13

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#16
Now what I'm going to say is basic stuff that everyone else here already knows, but maybe this will help the OP:

To figure out what the quarter wavelength of an antenna should be to be resonant for a particular frequency, you divide 234 by the frequency to get the length in feet. So here's the quarter wavelength for 853 Mhz:

234
----- = .275 or about 3"
853

Now let's say the quarter wave whip in your picture is for 40 Mhz. So the calculation would be:

234
----- = 5.85 or about 5' 10"
40

Perhaps this may help you understand why a 70" whip is not going to be resonant or effective at 853 Mhz.
Thank you for the explanation, I recently discovered they have low band conventual frequencies.


--
Best Regards,
kjl
 
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