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SWR and Power?

H

Highland Ranger

Active Member
Aug 28, 2005
141
9
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As some know from another thread, I just put a new Galaxy 959 in the truck.

Was DKing 6w with a Firestik cellular look alike mag mount

With a little wil, same spot on the roof, much lower SWR's (like from 2.2 down to 1.2) and it is now DKing higher at 10 watts.

Does that sound right?

Can the antenna or SWR effect transmit power as read on the radios meter? (realize it has effects on distance)
 

I'm just a truck driver (so I don't know this from a technical point of view), but I have read that if your SWR is too high, the radio's power is reduced. I do know that if your antenna is not matched correctly, you don't "get out" as well. I think it all comes down to efficiency. The better the antenna is tuned, the more efficient it is at getting your signal transmitted. I'm sure one of the tech's on here will step up and explain how a radio's power is stepped down when swr is too high (seems like I read the newer radios do this to try to protect the circuitry).
 
Yes, the antenna must be tuned so that the maximum possible signal is ging out. You don't have a tuned antenna. The SWR meter is usded to tune it. A high SWR WILL reduce your output power. It should be much lower, around 1.5 is a good SWR

You tune the antenna by moving the antenna whip up and down til you can get it as lower.
 
High swr will show more power on your meter but you will find that you don't get out as well(this is how a lot of cb shops out there show you those big numbers on there meters) Low swr will show you less power but you will find that you can get out better.As far as the power increase goes i would say that's because the coax on the little will is not very efficient and its causing the power to back feed and show you more power then what you are really doing. But hell what do i know,im just some old dumb trucker.
 
i will help make it simple...

low SWR's enable use of higher realised power
and saves the finals and other electronics of radio
(should be 1.5 or lower...i look closer if i get above 1.2 to 1)

high swr enables less of a tuned antenna
(or in other words an antenna not best suitted to the frequency you want and or need to transmit)

Low SWR equals an antenna with good to exelent transmittion
(known as resenot frequency.....where frequency resonates best at...and therefore transmitts best at....however an antennas best resodent frequency is not always the frequency it gets best swr....but usally it is)

many factors go into obtaining the best swr..
but that is perhaps another topic

Later
 
Highlander:

Yes, SWR can affect the reading you get for power. It can make it show either higher or lower depending on the phase relationships between the outgoing and reflected signal, and the specific design of the meter circuitry. The lower the SWR, the better your performance will be. However, you can get a low SWR reading on a meter and still have a mismatched antenna from impedences not matching up at the antenna end.

The length of the path that the signal has to travel from the meter to the antenna and back again will largely determine just how accurate your readings are. For CB channels, the correct lenght of coax to use while testing SWR is about 14.1 to 14.6 feet, or multiples thereof. Since the Little Wil comes with 17 feet, you may have some discrepencies in your reading, but nothing too bad. If you want to get picky, add an 11-foot section to you coax to test the SWR. When you're done, you can remove it with no ill effects.
 
Sorry dude..

Lenth of coax has nothing to do with SWR's..
at least not in how you describe..
whether coax is 14.1 feet of 20 feet is not going to make a desernable difference..

however 10 feet techniclly is better then 14.1 feet..
in other words..
use the shortest length you can..

do not suscribe to how so many foolishly say one must you coax lenths of whatever multibles..

Later
 
Coax length WILL affect what the SWR meter sees, NOT the actual antenna SWR. Take an antenna that is out of tune, and watch the SWR reading change as you shorten or legthen the coax. Take that same antenna, tune it properly, and then perform the same experiment. The meter reading stays the same as you change lengths now, not counting some minor fluctuations.

As for the 14.1 or 14.6 length, just do the math. Take a half-wavelength at a given frequency in freespace, multiply it by the velocity factor of the coax, and thats whats needed for the meter to see the same impedence as the antenna. Its called an impedence repeater. You can look it up if you want to. The ARRL's Antenna Handbook has all of that info and more.
 
coax

DTB is right if you want to try to give your swr meter a halfwave to read, but use what you ever length you need for your install. A 102 inch whip with a 6 inch spring shows good swr with just about all lengths of coax. As long as your antenna is resonant for the frequency you talk on you should have little problem. If you really get hung up on perfection you could get an MFJ antenna anylazer or tune with a bird meter for lowest reflect.
 

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