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Author Topic: newbie needs SWR help  (Read 1475 times)

Posts: 1

« on: September 09, 2004, 11:15:30 PM »

I am extremly new to this hobby. I am actually studying to get my ticket the 1st weekend on October. So that said I have began to play with some mobile 10 meter radios and cb radios. I have purchased a Solarcon I-max 2000 for 10-11 meter use. I figured this would be a inexpensive way to get my feet wet. The antenna is mounted on a 30' vertical pole attached to a metal fence post in my back yard. It seems to be grounded as there is continuity between the coax shield and the fence post. I have 100' of rg-8/u. I installed the antenna first without the radials and have a very low swr (1.1:1 or better) but only had about a 10 mile range on CB. I know some others who talk 30-40 miles with the same antenna so I decided to try the groundplane kit. After installing the kit I now have swr of 1.5:1 to 2.0:1. Is it common to see such a drastic change in swr when adding the radials? The antenna has tuning rings but I don't want to keep going up and down with this antenna as it reguires quite a bit of work for 1 person. Thank you for your help!


Posts: 303

« Reply #1 on: September 10, 2004, 08:13:20 AM »

Hello, and good luck to you on the test!

ANY metal object placed in the near field (esp. 8 ft. apx. with CB freq.'s) WILL have and effect on the impedance (thus SWR) of ANY antenna element. Sometimes it may be a small amount sometimes a large amount, this you will find out as you "read more about it", too much to go into here, but you would have to "re-tune"

The change in your SWR is NOT anything to worry about as far as the power loss vs. 1.1:1. The station on the other end will most likely not "hear" the difference as the dB change is small.

In short, what you are doing here with adding the radial kit is changing the PATTERN of the radiation off the antenna. As a 5/8 wave design, it's pattern in quite "flat", and as with this band, near field radiation results are with the ground wave, it is a good way to go.

Your horizon is a big factor here in comparing what results others get with the same set up. Terrain, total height (above sea level) ect.,. You may be at 30 feet with the mast, but if your ground height is only 1000', you won't get out like a station who starts out with his 30' foot mast at 1500' !!

Now with "sky wave" you will find as you learn more in the future it's another ball game. With this CB stuff, ground wave is the thing and these things I mentioned are the reality.

In short, what you will most likely see with adding the radials is a change at the other stations DEPENDING on where they are (in direction) in relation to you!

You’ll get some other views on this I’m sure. Sorry if I  don’t get the basic point to you as I’m in a hurry right now.




Posts: 21764

« Reply #2 on: September 10, 2004, 12:21:52 PM »

Luke, I understand from reading quite a bit about this particular antenna that it is not well decoupled from its feedline, by design: So, the outer shield of the coaxial cable is always "hot" with RF when transmitting.  If this is the case, then you should be able to change the SWR indicated on your meter down in the "shack" by simply running your hand up and down the cable, or worse still, by rubbing a bunch of steel wool (or anything conductive and flexible) along the outside of the cable.

The "radial kit" probably helps decouple the feedline a bit better from the antenna radiator, by providing a place for "I2" current to flow.  So, it should be a step in the right direction.  Whether the radials are really the right length to balance this vertical or not, I don't really know; however, any radials at all probably help.  As such, the SWR you see now is probably closer to what the antenna's really doing.

Regardless of what you may have heard on "CB" for years, SWR is not a bad thing!  It's just one of many things to consider when designing, building and installing antennas -- and rarely is it a very important one.  Most knowledgeable hams would much rather have a big, high, effective antenna with a 10:1 SWR than a small, low, ineffective antenna with a 1:1 SWR.  The big one, with the big mismatch, will work better.

However, since your antenna has its "tuning rings," now that you've added the radials, you can surely adjust those "rings" for the best possible match.  Problem is, best possible match on what frequency?  If the antenna's any good at all, it cannot possibly provide a perfect match at 27 MHz (CB) and also at 28.5 MHz (10m ham band).  If it does, that indicates it's pretty lossy, so the SWR "curve" is flattened out by antenna loss.  Not a good thing.

If you have no ham license, you shouldn't be transmitting on 10 meters, so you might want to adjust the "rings" for best SWR on CB for now, and then re-adjust them on 10m for when you get licensed with 10m privileges.  There won't be much difference in the way the antenna actually *works,* but you should be able to adjust the SWR for a "dip" on the band of your choice.

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