antenna matching

[Guest whitetaco.sg]

I am looking at the Midland - MXT575 Micromobile GMRS radio for off-roading, I however do not want a magnet mount antenna. my thought is to use a ditch light bracket as an antenna mount....... I can find all of the pieces to make what I want to do, the information I'm looking for is, does the coax length have a critical length requirement. E.G. citizens band has a formula to calculate coax length + antenna length = proper SWR   

[KAF6045]

On 5/24/2023 at 9:36 PM, Guest whitetaco.sg said:

I am looking at the Midland - MXT575 Micromobile GMRS radio for off-roading, I however do not want a magnet mount antenna. my thought is to use a ditch light bracket as an antenna mount....... I can find all of the pieces to make what I want to do, the information I'm looking for is, does the coax length have a critical length requirement. E.G. citizens band has a formula to calculate coax length + antenna length = proper SWR   

Proper coax is length neutral (it is 50ohm at the transmitter, and it is 50ohm at the antenna) -- that CB suggestion is just masking a problem by adjusting the coax so that the /nodes/ in the standing wave don't appear at the point of measurement. The SWR is still mismatched.

https://www.ad5gg.com/2017/06/11/coaxial-cable-length-does-not-change-swr/

Heck -- even ignoring standing wave nodes, if you use a long enough coax and measure at the transmitter you will achieve a perfect SWR... You won't be reaching anyone, but you will have a perfect SWR. SWR is based on the power sent to the antenna, and the power reflected from that antenna. With a very long coax, you have power losses in the coax itself, all the power sent/reflected could be lost as resistive heating of the coax so there is no measurable reflected power, but also no power going out the antenna.

The ideal is to measure SWR AT THE ANTENNA (or at least close enough that your body and measuring tools don't interfere with the feedpoint match). Of course, if the antenna has a permanently affixed cable you'd have to either learn to splice connectors into the cable or do the measurement from the end of the provided cable.

 

[OffRoaderX]

...

[OffRoaderX]

39 minutes ago, KAF6045 said:

Proper coax is length neutral (it is 50ohm at the transmitter, and it is 50ohm at the antenna) -- that CB suggestion is just masking a problem by adjusting the coax so that the /nodes/ in the standing wave don't appear at the point of measurement. The SWR is still mismatched.

https://www.ad5gg.com/2017/06/11/coaxial-cable-length-does-not-change-swr/

Heck -- even ignoring standing wave nodes, if you use a long enough coax and measure at the transmitter you will achieve a perfect SWR... You won't be reaching anyone, but you will have a perfect SWR. SWR is based on the power sent to the antenna, and the power reflected from that antenna. With a very long coax, you have power losses in the coax itself, all the power sent/reflected could be lost as resistive heating of the coax so there is no measurable reflected power, but also no power going out the antenna.

The ideal is to measure SWR AT THE ANTENNA (or at least close enough that your body and measuring tools don't interfere with the feedpoint match). Of course, if the antenna has a permanently affixed cable you'd have to either learn to splice connectors into the cable or do the measurement from the end of the provided cable.

 

Jeesuz.. "Some people" could over-complicate a spoon in their desperate attempts to show everyone how smart they think they are.

The answer, which unbelievably even after all those words was still not actually answered, is: NO

[WRXB215]

I enjoyed the answer that @KAF6045 gave, it was very informative. However, after reading it I did have to go back and re-read the original question.

OK @OffRoaderX, I'm setting the spoon down now before I hurt myself.

[axorlov]

The question was answered by the first 5 words. It's just that OffroaderX never learned to read.

[AdmiralCochrane]

9 hours ago, axorlov said:

The question was answered by the first 5 words. It's just that OffroaderX never learned to read.

I read your reply correctly on the first try