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

[ChrisL]

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

[berkinet]

...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

[berkinet]

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.

[WRAK968]

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.

[Lscott]

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.

[ChrisL]

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.

[ChrisL]

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.

[kipandlee]

1.5 swr is pretty good  changing to some hard line might get a little improvement  my last purchase of LMR-400  wasn't that good of cable    ( maybe a bad batch )

[Lscott]

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. 

[WRW999]

I know this is an old post but I wanted to ask a question regarding SWR since I just installed the Comet CA-712EFC. 

I mounted the antenna at the top of a 60ft Rohn 25g tower. I have a 5ft section of 1/2” superflex coax running from the antenna to connect to 7/8” hardline not yet installed. While checking SWR at the antenna today using a NanoVNA, I was surprised and disappointed to see a reading of 1.78 across 462.500-462.725mhz. One of the reasons I bought this antenna was because of countless reviews stating the low SWR around 1.1. 467-468mhz SWR was 1.2. This antenna will be used for a repeater, so only the 462mhz GMRS frequencies matter for my use. 
 

Prior to installing I used coax seal tape around the N-connector on the antenna and at the middle of the antenna where the two pieces lock together. While installing, I wasn’t paying attention and over tightened the bottom clamp and deformed the metal mount part of the antenna.
 

What gives? Are the great reviews regarding SWR coming from people using it as a base station transmitting on the higher 467mhz frequencies? 
 

I hate to have to bring the antenna back down to troubleshoot it. Any suggestions?

[SteveShannon]

4 minutes ago, WRW999 said:

I know this is an old post but I wanted to ask a question regarding SWR since I just installed the Comet CA-712EF. 

I mounted the antenna at the top of a 60ft Rohn 25g tower. I have a 5ft section of 1/2” superflex coax running from the antenna to connect to 7/8” hardline not yet installed. While checking SWR at the antenna today using a NanoVNA, I was surprised and disappointed to see a reading of 1.78 across 462.500-462.725mhz. One of the reasons I bought this antenna was because of countless reviews stating the low SWR around 1.1. 467-468mhz SWR was 1.2. This antenna will be used for a repeater, so only the 462mhz GMRS frequencies matter for my use. 
 

Prior to installing I used coax seal tape around the N-connector on the antenna and at the middle of the antenna where the two pieces lock together. While installing, I wasn’t paying attention and over tightened the bottom clamp and deformed the metal mount part of the antenna.
 

What gives? Are the great reviews regarding SWR coming from people using it as a base station transmitting on the higher 467mhz frequencies? 
 

I hate to have to bring the antenna back down to troubleshoot it. Any suggestions?

There are two different Comet antennas with similar numbers. One is the CA-712 EF and the other is the CA-712 EFC.  The EFC model is the GMRS one. Which one do you have?

[WRW999]

11 minutes ago, SteveShannon said:

There are two different Comet antennas with similar numbers. One is the CA-712 EF and the other is the CA-712 EFC.  The EFC model is the GMRS one. Which one do you have?

I have the EFC model. I double checked the receipt from DX-Engineering. Unfortunately, I dont have the packaging to verify the model number on it. 

[LeoG]

Mine runs 1.00:1 on all the repeater frequencies.  I've seen it as high as 1.04:1 which is still perfect.  What does it say at the base of the antenna, there is a sticker that will confirm if you have the C model or not.

Always possible you got a bad one, it happens.

Have you tried with a transmitter to see what a SWR meter says instead of just relying on the VNA?

[WRW999]

13 minutes ago, LeoG said:

Mine runs 1.00:1 on all the repeater frequencies.  I've seen it as high as 1.04:1 which is still perfect.  What does it say at the base of the antenna, there is a sticker that will confirm if you have the C model or not.

Always possible you got a bad one, it happens.

Have you tried with a transmitter to see what a SWR meter says instead of just relying on the VNA?

I’ll have to take a look tomorrow for the sticker you referenced. I only remember seeing the yellow warning sticker about electrocution, but I didn’t specifically look. I have a HT radio but my actual SWR meter isn’t battery powered and is a bit bulky to carry while climbing a tower. 

[LeoG]

Not a great picture of the sticker but shows you where it is.

Mine came in a CA712EF bag with a C sticker added.  I questioned the seller if it was correct and he said it came from the factory like that.  I was a bit surprised but then opened up the package and saw the sticker on the antenna and it was the C version.

[LeoG]

I knew I had a better picture somewhere.  Couple days after delivery.

[WRW999]

Thanks guys for the pictures of the label. I’m gonna be back on the tower tomorrow, so I’ll have to look. 
 

I do have hope after doing some testing with the NanoVNA and one of my actual SWR meters. I did a measurement of my home base GMRS antenna, a roll up J-pole and got the expected reading of 1.44. I then used the NanoVNA and got 1.9. I dont know why the NanoVNA is reading higher. I forgot all about having my Surecom SWR meter that I can easily take up on the tower with me tomorrow. I’ll take an HT and the Surecom and see what SWR readings I get. The real test will be after I get the 7/8 hardline installed which may actually be tomorrow or Monday. I can then take my go box and test with mobile radios. 
 

I’ll report back tomorrow! 

[SteveShannon]

5 minutes ago, WRW999 said:

Thanks guys for the pictures of the label. I’m gonna be back on the tower tomorrow, so I’ll have to look. 
 

I do have hope after doing some testing with the NanoVNA and one of my actual SWR meters. I did a measurement of my home base GMRS antenna, a roll up J-pole and got the expected reading of 1.44. I then used the NanoVNA and got 1.9. I dont know why the NanoVNA is reading higher. I forgot all about having my Surecom SWR meter that I can easily take up on the tower with me tomorrow. I’ll take an HT and the Surecom and see what SWR readings I get. The real test will be after I get the 7/8 hardline installed which may actually be tomorrow or Monday. I can then take my go box and test with mobile radios. 
 

I’ll report back tomorrow! 

Some SWR meters are incapable of measuring low power signals. So a very small reflected power won’t even be detected, resulting in a lower than actual SWR reading. I’m not saying that is what is happening.
The other thing is that the NanoVNA must be calibrated using three standards (Open, Short, and 50 ohms) in order to give its most nearly accurate readings.

[LeoG]

Don't you need to do a through to get the testing coax in the equation also?

[nokones]

1 hour ago, SteveShannon said:

Some SWR meters are incapable of measuring low power signals. So a very small reflected power won’t even be detected, resulting in a lower than actual SWR reading. I’m not saying that is what is happening.
The other thing is that the NanoVNA must be calibrated using three standards (Open, Short, and 50 ohms) in order to give its most nearly accurate readings.

A Bird 43 In-Line watt meter with a low power slug or a Telewave that has been recalibrated is a must for checking reflected power accurately.

[SteveShannon]

6 hours ago, LeoG said:

Don't you need to do a through to get the testing coax in the equation also?

I’m not 100% sure what you’re asking, sorry.  Testing the antenna and feedline together is important, yes, but it’s really good to know the SWR of the antenna at its feedpoint.  Unless there’s something really wrong or damaged with the feedline, the SWR of the combination of feedline and antenna (as measured at the radio end of the feedline) will always appear better than the SWR of the antenna measured at the antenna end. The normal attenuation that happens with every feedline makes the relationship between the forward power and reflected power look better at the radio than it really is.

So, by testing the forward and reflected power reading at the antenna and at the radio you can tell if your antenna is tuned correctly and also how much attenuation you have.  Then, you can accurately calculate your SWR at both places.

[BoxCar]

When calibrating your VNA, anything between the VNA and the test slug becomes calibrated into those readings.

[LeoG]

Usually the VNA has test cables for port 0 and 1.  You connect port 0 and 1 with that cable to take it out of the equation using the through function.  Mainly because you are testing a PL259 or N cable and the VNA connection is SMA so  you need an adapter and cable.

[nokones]

5 hours ago, SteveShannon said:

I’m not 100% sure what you’re asking, sorry.  Testing the antenna and feedline together is important, yes, but it’s really good to know the SWR of the antenna at its feedpoint.  Unless there’s something really wrong or damaged with the feedline, the SWR of the combination of feedline and antenna (as measured at the radio end of the feedline) will always appear better than the SWR of the antenna measured at the antenna end. The normal attenuation that happens with every feedline makes the relationship between the forward power and reflected power look better at the radio than it really is.

So, by testing the forward and reflected power reading at the antenna and at the radio you can tell if your antenna is tuned correctly and also how much attenuation you have.  Then, you can accurately calculate your SWR at both places.

Of course checking both ends for a base station application is not a problem, but try doing that with a mobile antenna installation could be somewhat complicated unless the antenna mount utilizes a SO239 connection.

[tcp2525]

On 9/13/2025 at 8:09 PM, WRW999 said:

I have the EFC model. I double checked the receipt from DX-Engineering. Unfortunately, I dont have the packaging to verify the model number on it. 

Needn't worry, my first one was a dud as well, it was a great attenuator instead af a gain antenna. Just contact them and they will make it right. They probably sold you my returned item. Just kidding, they were great and the replacement kicks butt. But in all seriousness, that antenna's SWR meets published specs and they might laugh at you. 

[WRQF617]

On 9/13/2025 at 6:08 PM, LeoG said:

Not a great picture of the sticker but shows you where it is.

Mine came in a CA712EF bag with a C sticker added.  I questioned the seller if it was correct and he said it came from the factory like that.  I was a bit surprised but then opened up the package and saw the sticker on the antenna and it was the C version.

Mine came the same way and I questioned that as well!