Cable length and SWR

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wbswetnam

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I recently bought a mag mount scanner antenna (Radio Shack cat # 20-032) which came with 16' length of coax. If I cut the coax length in half and put a twist-on PL-259 plug on the end, will this significantly affect the overall SWR characteristics of the antenna system? Also, could it do double-duty as a 2m ham antenna?
 

w2xq

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No and no. Length won't affect receive. It's a wide-band receive antenna. HTH.
 

n5ims

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I recently bought a mag mount scanner antenna (Radio Shack cat # 20-032) which came with 16' length of coax. If I cut the coax length in half and put a twist-on PL-259 plug on the end, will this significantly affect the overall SWR characteristics of the antenna system? Also, could it do double-duty as a 2m ham antenna?
No, SWR is power going out vs. power being reflected back from a mismatched antenna. The length of the coax will have nothing to do with how mismatched that antenna is. One thing that confuses most folks is that many SWR meters (generally the lower cost ones - note lower cost refers to design and manufacture, not what you pay) are designed not to read the actual power levels, but the voltage/current levels which vary according to where you measure them. This is because they vary from positive to negative values (a sine wave) from the transmitter out and the antenna back. The voltage/current level you measure will depend on what electrical length (in wavelength based on the transmitted frequency) you measure it from. Select the right point and there will be no voltage and select another point and there will be maximum voltage.

The reviews indicate that you can use it to transmit, but only at fairly low power levels. The SWR may not be great on 2 since it appears to be resonant around 157 MHz (per the poster that used their antenna analyzer on it). Might be OK using a HT, but a mobile unit may exceed the power ratings.
 

LtDoc

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Very simply, if you are going to use the thing on 2 meters buy a 2 meter antenna. It'll work fine on a scanner. SWR isn't a biggy with scanners or for just receiving. It is a biggy (sort of) when transmitting. Lots of 'buts' in that, but generally true.
- 'Doc
 

AgentCOPP1

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Like others have said, no. The antenna is the main determining factor of SWR, not your cable. Now if your coax becomes any fraction of a wavelength of what you're using (if transmitting), it could potentially cause problems to occur, but if you're just receiving, you don't need to worry about that.
 

majoco

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Stop listening to the CB-ers who think you can tune the antenna to minimum SWR by trimming the length of the coax!
Yes, you can get down to 1:1 SWR at the transmitter by trimming the coax. But it won't improve your transmit coverage or your receiver sensitivity and will only be true for one frequency - and you could have a high voltage somewhere in the coax which may go splat one day....
Tune the antenna properly and you can have any length of coax you like.
 
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