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Lowering SWR on Comet Original CA-712EFC 460-470Mhz


ChrisL
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Hi all,

 Got my antenna up and running after about 20 hours worth of work (not including Home Depot trips).

 First I want to mention the performance was better than I had hoped for. Very happy with that. Yesterday had a long chat on a repeater that I wasn't even expecting to hit.

 The second thing Is the the SWR was 1.5 on Medium power from my BTECH 50x1 and and 1.3 on low power. I would like to improve on this.

 I'm using 33ft. of  Times MicroLMR-400 coax with Amphenol PL-259 and Time Micro N-male connector. The #4awg ground rod wire is about 4ft long. The antenna is about 5 ft. over the top of the roof line. I'm using a Diamond 400 SWR meter.

 The instructions said to use either 1 or 2 loops for the rf choke. I made one loop. The radials have a VERY small adjustment of about 1/4" on all 3. I set the radials in as far as they would go and tightened things up. So I could extend them a bit.

 Would adding another loop to the rf choke help and would lengthening the 3 radial make a difference?

 Any advice would help.

 I forgot to mention that I tested the SWR on all the GMRS and GMRS repeater frequencies and they where all very close in measurement.

 Thanks guys for all your help.

 Chris

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...There is a house next to the granny house this is mounted on about ten feet away and it's roof is slightly taller than the roof the antenna is attached. There aren't any metal poles nearby but there is an orange tree.

 Speaking of ground plane radials. I was wondering if simply turning the antenna to change the direction the radials are pointed could change the swr? I wouldn't even need to take down the antenna to turn it by a few degrees.

The objects you n note may affect your overall transmission, but are fairly faraway and unlikely to impact SWR. As for the ground plane... The keyword here is plane, as in geometry, a flat surface. Your radials are creating the effect of a ground plane without actually creating the full plane. More radials would make the ground plane a but more effective, as would a solid metal disc. But, the difference would be negligible.

 

However, keep in mind what I and others have noted above: 1.5:1 is quite good. You could spend a lot of time, and money getting it better and probably see no effective difference.

 

Given the kinds of questions you are asking, I would suggest taking a look for some ham radio license study guides on antennas.  You don't need to go the full route for a ham license (though that wouldn't be a bad idea). Just look for some basic on antennas

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You are already in pretty good shape. Dropping below 1.5, even to 1.0 won't make a substantial increase in performance. It's not nearly as noticeable as, say, going from 2.0 down to 1.5. But, if you want to try for some small improvements...

 

In general, I don't think you can have too much ground plane. (And I am sure someone here will correct me if that is wrong.)

So, I'd extend the ground radials as much as possible and see if that makes any difference.

 

Also, you don't mention it... but, are there any other vertical metal poles or other antennas anywhere close by? Other objects might affect your transmission and SWR.

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What is the antenna mounted on? Metal pole? Fiberglass? PVC? All of these can affect SWR from my experience. Switching from fiberglass to a 6 foot metal fence post lowered SWR from 1.7 to 1.08. Another thing that affects the SWR reading is the length of feedline. I was told to make sure you measure SWR at one and a half wavelengths, which if I recall is 18 inches from the radio. Many people overlook that and get crazy readings which cause them to panic.

Overall a 1.5 is reasonable (anything 1.5 or less is near perfect) I've run radios as high as 1.7 with little issue. I wouldn't run anything over 2.0 though.

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I would NOT recommend using PL-259 and SO-239 connectors at UHF. They are not "constant" 50 ohm impedance. Depending on how many you have and where in the system they do tend to degrade the SWR. Good connectors to use at UHF are "N" type, BNC, mini RG8, SMA are the more common ones.

 

Many mobile and base radios use an SO-239 on the back so you can't do much about it. The better ones use the "N" type. Where you see SO-239's used look at the inside of the connector. If you see what looks like thin a web between the outside of the center pin and the inside of the outer shell, like the spokes on a wheel, cause less of a problem compared to the solid filled ones. This is done to improve the impedance of the connector to bring it closer to the desired 50 ohms.

 

The antenna ground radials, if required, should be around 1/4 wave length long, approximately 6 inches long, and could be bent downward at a 45 degree angle from the horizontal.

 

Some of the antennas that claim not to require a ground plain are a "J-Pole" internally. These antennas should have several turns of the coax located right at the base to decouple it from the outside of the coax shield. If this isn't done then RF current flows on the outside screwing up the antenna TX and RX pattern, RF getting into the radio resulting in TX audio distortion and in addition to causing weird SWR problems. 

 

If your SWR is 1.3 to 1.5 there may not be much to gain by improving it to the ideal 1 other than to make one feel better. Most radios are designed to tolerate SWR's up to 2  at full power anyway.

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What is the antenna mounted on? Metal pole? Fiberglass? PVC? All of these can affect SWR from my experience. Switching from fiberglass to a 6 foot metal fence post lowered SWR from 1.7 to 1.08. Another thing that affects the SWR reading is the length of feedline. I was told to make sure you measure SWR at one and a half wavelengths, which if I recall is 18 inches from the radio. Many people overlook that and get crazy readings which cause them to panic.

 

Overall a 1.5 is reasonable (anything 1.5 or less is near perfect) I've run radios as high as 1.7 with little issue. I wouldn't run anything over 2.0 though.

Thanks for the response. The antenna is mounted on two sections of top rail (metal) and the coax goes into the shack through a pass through in the wall. I'm using a one foot jumper made of LMR-240.

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You are already in pretty good shape. Dropping below 1.5, even to 1.0 won't make a substantial increase in performance. It's not nearly as noticeable as, say, going from 2.0 down to 1.5. But, if you want to try for some small improvements...

 

In general, I don't think you can have too much ground plane. (And I am sure someone here will correct me if that is wrong.)

So, I'd extend the ground radials as much as possible and see if that makes any difference.

 

Also, you don't mention it... but, are there any other vertical metal poles or other antennas anywhere close by? Other objects might affect your transmission and SWR.

Thanks again for your help. There is a house next to the granny house this is mounted on about ten feet away and it's roof is slightly taller than the roof the antenna is attached. There aren't any metal poles nearby but there is an orange tree.

 Speaking of ground plane radials. I was wondering if simply turning the antenna to change the direction the radials are pointed could change the swr? I wouldn't even need to take down the antenna to turn it by a few degrees.

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Actually going to a lower loss line could make the SWR worse as seen from the radio end. The reason why is the energy reflected from the antenna back to the radio is attenuated less, more reflected power, that would otherwise be adsorbed by the feed line.

 

The SWR can be calculated using forward and reflected power using the formula below:

 

SWR = (1+sqr(Pref/Pfwd))/(1-sqr(Pref/Pfwd)) 

 

Where "sqr()" is the square root of the value inside of the "()". And "Pfwd" is the measured forward power going to the antenna while "Pref" is the power being reflected by the antenna miss-match back to the radio. 

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