Antenna tuning

[Guest MManuel]

I'm not much of an RF guy, hence my interest in GMRS vs HAM, so I'll pose y'all a Newbie question in hopes of learning something.  How do you tune an antenna for GRMS, is it the same as with a CB antenna where you use a SWR meter and lengthen/shorten to achieve the lowest reading across a spectrum of frequencies.  If so, do I use the same or different kind of SWR meter than a CB?  I'm thinking its the same process, same meter, just making sure that the power is on low when testing, am I wrong? 

[BoxCar]

Same process, different meter.

[GuySagi]

And I don't know if my experience is unique, but I was able to get my CB SWR down to close to 1:1 across all channels. But that's only somewhere around 1 mhz of separation...so tune it to 20 and everywhere on the dials happy. 

 

On GMRS I'm pulling my hair out and pretty sure I'll be setting for a few channels sucking at 1:5. I'm probably wrong, but my excuse  right now is there's a total of 5 mhz separation on this service. And I didn't exactly go frugal on the setup, either. 

[n4gix]

You can NOT use a CB SWR meter... Period.

 

Quit tearing your hair out.

[WRAK968]

And I don't know if my experience is unique, but I was able to get my CB SWR down to close to 1:1 across all channels. But that's only somewhere around 1 mhz of separation...so tune it to 20 and everywhere on the dials happy. 

 

On GMRS I'm pulling my hair out and pretty sure I'll be setting for a few channels sucking at 1:5. I'm probably wrong, but my excuse  right now is there's a total of 5 mhz separation on this service. And I didn't exactly go frugal on the setup, either. 

Most UHF radios can handle up to 2.0 SWR, (Actually most radios can handle up to 2.0 with no damage) 1.5 is great and 1.0-1.1 is near perfect. For GMRS tuning, if your only using the 462 frequencies you would tune it at 462.6250. If your tuning for repeater operations you should tune it for 465.125. (Use that frequency for where to cut the antenna then fine tune. with the SWR meter.) One thing to note as you tube is to check both 462 and 467 frequencies BEFORE making cuts/adjustments and if you have a reasonable reading on one and near perfect on the other you should stop.

[GuySagi]

I bought and am using a UHF SWR meter for the task N4gix...sorry for the confusion in my reply/post. Good reminder for everyone, though, because I've messed up on some simple things by not knowing or investing the time to double check. And WRAK968, your tip/reminder is a timely one in my case. I finally found a repeater I can hit from the house during the next hurricane/tropical storm (assuming it's still on the air). So I'll be checking I can hit it, then tuning for that frequency—would have meant unneeded additional work if I tuned for the middle of the band, put the antenna and mast up and then came up with that approach. Now if I can clear the rest of the household/yard priorities off my list...

[berkinet]

... I finally found a repeater I can hit from the house during the next hurricane/tropical storm (assuming it's still on the air). So I'll be checking I can hit it, then tuning for that frequency...

 

You may just wish to follow WRAK968's suggestion. Because of the 5mHz rx/tx offset, you are still going to be tuning somewhere in the 465.xxx range anyway. That is, unless you are sure you will never want to use simplex and you are willing to slightly sacrifice reception for transmission. Keep in mind: Perfect is the enemy of good

[JohnE]

I bought and am using a UHF SWR meter for the task N4gix...sorry for the confusion in my reply/post. Good reminder for everyone, though, because I've messed up on some simple things by not knowing or investing the time to double check. And WRAK968, your tip/reminder is a timely one in my case. I finally found a repeater I can hit from the house during the next hurricane/tropical storm (assuming it's still on the air). So I'll be checking I can hit it, then tuning for that frequency—would have meant unneeded additional work if I tuned for the middle of the band, put the antenna and mast up and then came up with that approach. Now if I can clear the rest of the household/yard priorities off my list...

the truth of the matter is "most" base antennas will have a "range" that they are good for (i.e. 460Mc to 470c) there is little or in most cases no tuning required.

as to mobile antennas that is a bit different. Unity(0 gain 6" whip) antennas do not need tuning. The 3dB and 5dB will have to be "cut" depending on brand but there are a few that will be good to go out of the box.

I also agree w/WRAK968's suggestion. I don't know what meter you are using but a 2:1 swr is 10% of forward power and 1.5:1 would be 4% of forward power.

100 forward 10 reflected would be a 2:1

100               4                                    1.5:1

as an example

 

[GuySagi]

Great advice you guys. Now that the darned driveway is resealed maybe I can get back on it this week! Hope everyone has an awesome and safe week, by the way. 

[jaycee]

I have a follow-on question to this string.  I know how to tune for SWR and have the right meter, my question is where to tune the antenna?  Do you tune it sitting on the ground, on the workbench, on a mast 20 feet up?   Is there a method for doing this so you get accurate readings and not grounded unnecessarily?  I've done a base antenna one time but it was up on the mast and I took it up and down a few times to get it right.  Can this be done laying down on the mast?  

[MacJack]

So I'm ready for a new UHF SWR meter, any suggestions.

[BoxCar]

So I'm ready for a new UHF SWR meter, any suggestions.

Depends. What do you have? I bought a Surecom digital from everyone's favorite online store.

[gortex2]

So I'm ready for a new UHF SWR meter, any suggestions.

 

Just went thru this....

 

https://forums.mygmrs.com/topic/2267-good-swr-needle-meter/?do=findComment&comment=21105

[Lscott]

From experience the best way to tune your antenna is when it is installed in its final location. Near by metal objects, height above ground etc. tend to affect the antenna. It might not be that convenient but you’ll know the match won’t change because you move the antenna location.

[MacJack]

So is that Amazon?  Thanks.

Depends. What do you have? I bought a Surecom digital from everyone's favorite online store.

[MacJack]

Yes, but getting more input on this post...  thinking digital read better than needle.  

Just went thru this....

 

https://forums.mygmrs.com/topic/2267-good-swr-needle-meter/?do=findComment&comment=21105

[MacJack]

Forgive me if I'm off subject on this post.... as a newbie, I value your comments on the following:

 

I have a Midland MicroMobile MXTA26 6db Gain Whip Antenna and want to use it on my Jeep and RV...  In my Jeep will roll up the extra antenna cable under the hood but in my RV will extend it all it out...  I'm using a Midland MicroMobile MXTA12 Antenna Mag Mount with a 12 foot cable.  I also plan to use this antenna for a future purchase of a Retevis RT97 Portable GMRS Repeater 5W   https://shop.mygmrs.com/collections/featured-products/products/retevis-rt97-gmrs-repeater-5w  while Jeeping, camping, hunting for my HT.  

 

So do I have to retune the antenna for the Jeep, RV with my HT as well as the Retevis RT97 Portable GMRS Repeater 5W..

 

Thanks in advance for your patiences. 

 

Jack

[mbrun]

There is nothing wrong with verifying tuning is still in acceptable range when using on both vehicles, but I do not expect it to be something that will require any recurring action on your part.

 

No, you do not need to retune an antenna when you switch the radio you use. Tuning is a function of the frequency of operation and the physical installation of the antenna. As long as the antenna is used with only GMRS frequencies you can use whatever radio you want.

 

Forgive me if I'm off subject on this post.... as a newbie, I value your comments on the following:

 

I have a Midland MicroMobile MXTA26 6db Gain Whip Antenna and want to use it on my Jeep and RV... In my Jeep will roll up the extra antenna cable under the hood but in my RV will extend it all it out... I'm using a Midland MicroMobile MXTA12 Antenna Mag Mount with a 12 foot cable. I also plan to use this antenna for a future purchase of a Retevis RT97 Portable GMRS Repeater 5W https://shop.mygmrs.com/collections/featured-products/products/retevis-rt97-gmrs-repeater-5w while Jeeping, camping, hunting for my HT.

 

So do I have to retune the antenna for the Jeep, RV with my HT as well as the Retevis RT97 Portable GMRS Repeater 5W..

 

Thanks in advance for your patiences.

 

Jack

 

Michael

WRHS965

KE8PLM

[MacJack]

Thanks.  

There is nothing wrong with verifying tuning is still in acceptable range when using on both vehicles, but I do not expect it to be something that will require any recurring action on your part.

No, you do not need to retune an antenna when you switch the radio you use. Tuning is a function of the frequency of operation and the physical installation of the antenna. As long as the antenna is used with only GMRS frequencies you can use whatever radio you want.



Michael
WRHS965
KE8PLM

[jaycee]

From experience the best way to tune your antenna is when it is installed in its final location. Near by metal objects, height above ground etc. tend to affect the antenna. It might not be that convenient but you’ll know the match won’t change because you move the antenna location.

 

And I'm good to tune this in the final position it will be in.  It is more of a question of what is the right method.  I don't want to tune it and then find out I cut off too much of something and then it's ruined.  Thanks. 

[Lscott]

And I'm good to tune this in the final position it will be in.  It is more of a question of what is the right method.  I don't want to tune it and then find out I cut off too much of something and then it's ruined.  Thanks. 

Most antennas allow some adjustment. That's usually done by loosening a set screw that holds the whip element in place. 

 

What I've done is set the whip down as far as it will go then check the match. If it's too high, and you need to know if the match gets better as you move up in frequency towards the desired mid-band point, or you move down in frequency.

 

If it looks like the whip is too long, the match improves as you move down in frequency to the mid band point, the whip is too long. Try cutting off about a 1/16 inch off the bottom of the whip then repeat the measurement. At the frequencies use by GMRS you have to be careful to only cut off very small lengths of the whip.

 

If the whip is too short just loosen the set screw and move the whip up a small amount. Then recheck the match.

 

On gain type antennas they are normally pre-tuned by the manufacture. Other than the small adjustment in whip height allowed by the set screw it's recommended not to cut the whip. There are several sections on the gain antennas separated by phasing coils. Cutting the whip will likely make things worse since each section would need adjustment.

 

The best and cheapest antenna is a plain old 1/4 wave whip, about 6 inches long. It has the widest band width. I build several out of heavy bus wire and a PCB BNC connector, a 1/4 wave antenna with 4 ground plane elements, as an experiment. I got a 2:1 max SWR over an approximate 430 MHz to 470 MHz range with the min around 450 MHz. The antenna is usable for both the Ham 70cm band and GMRS.

 

For GMRS I would look for the min SWR at 465.144 MHz, which is half way between the lowest simplex frequency and the highest repeater input frequency.

[BoxCar]

Measure twice, cut once. A lot of antennas have a little slop at the base which allows the antenna to be made longer. Take a reading with the antenna fully seated in the base then another as you raise the antenna about a 1/4 inch in the base. see which way the SWR changes.  That will tell you if you need to lengthen or shorten the antenna. If shortening, cut off about 1/4inch increments and work from there.

[Harlanwillis]

When trimming a mobile antenna for the lowest reflected power I use a trick learned many years ago. At some point additional trimming will cause the reflected power to start to increase. The whip is beginning to be too short at this point. When unsure if you have reached the ideal length, I temporarily make the whip a little bit longer by adding a small piece of wire to the top of the whip where the little ball is attached. A short piece of solder works great. Wrap a couple of turns around the whip and extend the solder past the ball on the end about a quarter of an inch. Measure the reflected power/swr. If it improves or stays about the same stop any additional trimming. You are about as good as you can get it. If adding a small amount of solder causes the swr/reflected to go back up then you are on the right track. Continue trimming a small amount at a time. If removing a sixteenth of an inch from the whip fails to make any improvement then stop. Probably as good as you can get it.

[jaycee]

Most antennas allow some adjustment. That's usually done by loosening a set screw that holds the whip element in place. 

 

What I've done is set the whip down as far as it will go then check the match. If it's too high, and you need to know if the match gets better as you move up in frequency towards the desired mid-band point, or you move down in frequency.

 

If it looks like the whip is too long, the match improves as you move down in frequency to the mid band point, the whip is too long. Try cutting off about a 1/16 inch off the bottom of the whip then repeat the measurement. At the frequencies use by GMRS you have to be careful to only cut off very small lengths of the whip.

 

If the whip is too short just loosen the set screw and move the whip up a small amount. Then recheck the match.

 

On gain type antennas they are normally pre-tuned by the manufacture. Other than the small adjustment in whip height allowed by the set screw it's recommended not to cut the whip. There are several sections on the gain antennas separated by phasing coils. Cutting the whip will likely make things worse since each section would need adjustment.

 

The best and cheapest antenna is a plain old 1/4 wave whip, about 6 inches long. It has the widest band width. I build several out of heavy bus wire and a PCB BNC connector, a 1/4 wave antenna with 4 ground plane elements, as an experiment. I got a 2:1 max SWR over an approximate 430 MHz to 470 MHz range with the min around 450 MHz. The antenna is usable for both the Ham 70cm band and GMRS.

 

For GMRS I would look for the min SWR at 465.144 MHz, which is half way between the lowest simplex frequency and the highest repeater input frequency.

 

Great info.  So after cutting a few times, mid-range to the SWR is at 2.0 now.  1.7 at the lowest GMRS channel and 2.6 at the top of the repeater channel. It was much worse when I started but doesn't seem to be going down any further than this.  If this was pre-tuned at the manufacturer, then I think they missed the mark.  The other two antennas I got I had to do no cutting and they are fine.  Maybe this is just a bad one.  

[jaycee]

When trimming a mobile antenna for the lowest reflected power I use a trick learned many years ago. At some point additional trimming will cause the reflected power to start to increase. The whip is beginning to be too short at this point. When unsure if you have reached the ideal length, I temporarily make the whip a little bit longer by adding a small piece of wire to the top of the whip where the little ball is attached. A short piece of solder works great. Wrap a couple of turns around the whip and extend the solder past the ball on the end about a quarter of an inch. Measure the reflected power/swr. If it improves or stays about the same stop any additional trimming. You are about as good as you can get it. If adding a small amount of solder causes the swr/reflected to go back up then you are on the right track. Continue trimming a small amount at a time. If removing a sixteenth of an inch from the whip fails to make any improvement then stop. Probably as good as you can get it.

 

Interesting trick, I hadn't done that before but will give it a try to see what it does at least.  Thanks.