SWR question

[Runaway]

I just read somewhere that I should check SWRs at no less than 2 watts dead key....I have always set them with my radio on low ( around 1 watt DK ) with amp out of line....Is this the right way?? I know before I go and check that they will be high with my radio on high....

 

[W5LZ]

Runaway,
If the SWR is very high there are some quite large voltages on the feed line/antenna/etc. The higher the power during an SWR check, the larger those voltages can be. So, to minimize any possible problems from that high voltage possibility, SWR is usually checked using low power. How much is low power? That just depends, but usually as low as is practical considering the equipment you happen to be using. If 5 watts is as low as it'll go, then that's just fine. If you can get down below that, that's fine too. If you can't get the power output very low (less than 100 watts), just keep the test as short as possible.
The power numbers used, the 1, 5, or 100 watts, are only for examples. 'Barefoot', or 'stock' power outputs are okay. SWR is the same no matter what power level it's checked at. The idea is to prevent damage to the system while checking it.
- 'Doc

 

[Runaway]

Thanks Doc, but i'm not sure I understand,but it sounds like you are saying the way i'm doing it is ok...I check it really low and can get it down to like 1:1 and 1:2 on low power....So this is ok?...It goes up with like 5 watts DK....Im sorry I don't get it better....

 

[Beetle]

The diodes in some SWR meters prevent accurate readings at very low power levels. You have to overcome this non-linearity, and the only way to do it is to use a higher power or a more expensive meter. Once you're up around 4 watts you should be good to go. Keep the transmissions short.

 

[loosecannon]

just an opinion,
i think you should check your SWR at the power level you intend to operate at. understandably; you would first set the antenna for the lowest SWR with nothing but the SWR meter inline, but after tuning the antenna, and putting all the gear inline, the SWR should be checked again with the amp on.
THIS is when you find the other problems in your system that cause high SWR readings.

i have personally had experiences with bad coax connections, and bad antenna connections that seemed fine at 4 watts, but throw 100 watts at them and all hell breaks loose.

and dont forget about SWR going into the amp!
loosecannon

 

[W5LZ]

"Beetle"s right, some meters just don't do well at very low power levels. The solution to that is to just use a little more power, something around 5 watts, give or take a few. There's not really all that much danger of harming anything at that power level, depends on what you are running, of course, but typically 5 watts ain't gonna hurt 'nuthin'.
There should be no difference in SWR at 5 watts, or 50 watts, or 500 watts, if the equipment used can handle that power level. SWR is a ratio, so it stays the same unless that ratio changes. Meters are not all created 'equal', and don't always give the same accuracy in readings at different regions in their scales. There shouldn't be really significant differences though. A few 'points' difference in the 'tenths' position are just not that significant. If the number in front of the decimal place changes, then that is significant and something just "ain't right". For those meters that require calibrating, recalibrating when changing power levels is a smart thing to do.

Having said all that, I'll add this to complicate things just a bit. SWR readings are one of the least useful things in knowing how well an antenna system if working. Some antennas with 'not good' SWR do much better than some antennas that are absolutely 'flat' (1.0:1).
- 'Doc