SWR-RADIO Specific?

[Clubguy]

Is SWR tuning radio specific or can I set up my antenna and radio for SWR, then switch radios and have the same SWR or will I need to tune again? This could be a deciding factor on buying a new antenna now and waiting to upgrade radio until a few months later.

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[w8prr]

SWR is a function of antenna, some feed line effect and radio frequency. In theory it should be the same for any radio, although some radios may have a slight difference form others.

 

[K7MEM]

I agree with W8PRR. Most radios are designed with a 50 Ohm output in mind. So, yes, if the "only" thing you swap out is the radio, there should be very little difference in the SWR measurement.

SWR, as measured from the radio end, is a measurement that indicates the impedance match, or mismatch, between the antenna and the feedline. A much better idea of the antenna impedance can be had by measuring directly at the antenna. This removes the coax losses from the reading. But this is not always convenient or possible.

Measuring SWR from the radio end can hide problems with the antenna. For example, if you measure SWR from the radio end, and then upgrade your coax from say RG-58 to RG-8. It is very likely that your SWR will increase. The high losses in the lower grade cable, RG-58 in this example, may let you see a low SWR than really exists. But when you reduce the cable losses, by upgrading the coaxial cable, a higher SWR might show up.

So it would be best if you tuned your antenna by using the best and shortest coax available.

 

[Clubguy]

I'm not following you point about better coax having a higher swr.

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[rivardj]

The SWR we commonly discuss is actually VSWR which stands for Voltage Standing Wave Ratio which is the ratio of an AC voltage due to standing waves along the transmission line versus that of the minimum AC voltage along that line. If the SWR meter sees less reflected power, the SWR presented to the radio is lower. In the case of lossy feedline some of the reflected power is absorbed in the feedline instead of all being reflected back to the radio.

What K7MEM is saying that the lower quality coax will attenuate the reflected power of the standing waves before it reaches the radio end of the feedline, which will show some SWR value at the radio end. When higher quality coax is used, the reflected power will not be attenuate as much in the feedline thus causing a higher SWR reading to be present at the radio end due to higher reflected power reaching the radio end of the line. All this happens without the impedance match at the antenna end of the feedline changing. The antenna system will be any more efficient in radiating a signal with either feedline, what changes is where the power is dissipated within the system. VSWR is not the end all be all number to look at when evaluating antenna system performance.

 

[KC4RAF]

Agree with K7MEM and rivardj posted.

Think like this:
Use the VSWR meter at the antenna to see what it reads. This way you get a picture of the ANTENNA'S actual match or mismatch to the radio. If the antenna has a good match, then add your coax, (and always try to get the best you can afford). Then take a reading with the VSWR meter at the TRANSMITTER. This way you get a picture of just how well the coax is transferring the signal to the antenna. If there's a problem, you know it's the coax or connectors at fault.
HTH. I know it all sounds confusing, but it's the best way to read the antenna's and the cable quality.