swr cord length

[SPRINTERLIGHT]

Hey Everybody!!!!!!!!!
Question , I am using a 9 foot coaxial cable to check my swr
My readings are about 2 If I use a shorter cable will this lower them
Thanks

 

[teufler]

on vhf , I use multiple lengths of 18" increments. what frequency are you checking. VHF or CB?

 

[majoco]

No. If you have a correctly tuned antenna resonant and matched to the frequency in use and the correct impedance coaxial cable to the transmitter then the length of the cable makes no difference whatsoever. The only difference that the length of cable can make is that if the cable is so long that it attenuates the transmitted power then the returned power from the mismatched antenna is reduced too and the VSWR actually looks better.

 

[jonwienke]

My readings are about 2 If I use a shorter cable will this lower them

No. It will appear to, but the only way to increase actual output from the antenna is to properly tune it to the TX frequency. Coax length should always be the shortest possible to get the job done.

 

[jackj]

SWR stands for Standing Wave Ratio and is measured using a Standing Wave Bridge. A SWR meter measures the ratio of the standing wave voltages, not power. The proper name for it is a VSWR meter. If your antenna is not properly tuned then you can change the reading by changing the length of cable between the output of the transmitter and the bridge but the amount of reflected power will remain the same. That is one of the reasons why a SWR bridge is not an accurate way to determine antenna match. The easiest way to accurately measure antenna match is by using a directional watt meter and comparing the forward power to the reflected power. Tune your antenna for the lowest reflected power coupled with the highest forward power.

 

[SPRINTERLIGHT]

Thanks for the confusion!! All I wanted to know is Should the length of coaxial from the receiver to the swr meter be as short as possible. I am just curious if it effects anything , and I am not trying to lower my swr reading with changing coaxial length. The coaxial is 9 feet long
Thanks

 

[JackdAnderson]

short answer....no.

 

[SPRINTERLIGHT]

Thank you Jack Anderson!!!!!!!!!!!

 

[zz0468]

The meter sees the entire length of coax between it, and the antenna. If you're adding 9 feet to an existing coax, then you've made a random length of feedline between the antenna and the meter. Any measurement made that way will be random, and therefore in error.

If the TOTAL line length between the meter and the antenna is 1/2 wavelength exactly, the meter will see the same impedance as exists at the antenna end of the cable. The meter will still be in error because of line loss, but at least the complex impedance will be the same.

At CB frequencies, 9 feet is 1/4 wavelength, which will invert the impedance between the meter and the antenna, and SWR meter readings will in error.

The only accurate way to measure antenna SWR with an SWR meter is to place the meter at the antenna terminals. Anything else will introduce errors. At CB frequencies, 9 feet of cable will maximize the errors. In that case, the only time the meter will read correctly is if the antenna SWR is a perfect 1:1.

 

[prcguy]

Yes, but you didn't mention the need to calculate the 1/2 wavelength of coax including the velocity factor of the coax, which will make the coax test jumper shorter than a 1/2 wavelength in free space.

For most HF use a 1ft jumper is so small it will disappear in the measurement but at CB it would have a slight effect, if the jumper will have any effect at all. In most situations you probably won't notice any change with different length jumpers but if the antenna has a lot of RF on the coax (common mode currents) where the feedline is part of the antenna by poor design, then its more likely to be sensitive to jumper lengths.
prcguy

The meter sees the entire length of coax between it, and the antenna. If you're adding 9 feet to an existing coax, then you've made a random length of feedline between the antenna and the meter. Any measurement made that way will be random, and therefore in error.

If the TOTAL line length between the meter and the antenna is 1/2 wavelength exactly, the meter will see the same impedance as exists at the antenna end of the cable. The meter will still be in error because of line loss, but at least the complex impedance will be the same.

At CB frequencies, 9 feet is 1/4 wavelength, which will invert the impedance between the meter and the antenna, and SWR meter readings will in error.

The only accurate way to measure antenna SWR with an SWR meter is to place the meter at the antenna terminals. Anything else will introduce errors. At CB frequencies, 9 feet of cable will maximize the errors. In that case, the only time the meter will read correctly is if the antenna SWR is a perfect 1:1.

 

[zz0468]

Yeah. I neglected to mention velocity factor in the coax.

 

[robertmac]

Well as this is a scanner/receiver forum and not transceivers, my assumption [and I know about assumptions] then if only receiving or scanning, length would not matter. Others have responded if it is a transceiver.

 

[W6KRU]

Deleted.

 

[jackj]

Robertmac, what has an effect on a transmitter will also have an effect on a receiver. Otherwise the loss in the coax would not have any effect on the received signal.

Sprinterlight, I answered your question. Sorry if I confused you with facts but the short answer is, yes, it will have an effect on the reading and no, it won't effect the actual SWR.

 

[SPRINTERLIGHT]

Thank you to all and to all a good night!!!!!! HO HO HO