Easy Way To Tune A GMRS Antenna

If you’ve installed a GMRS radio in your overland vehicle, you’ll probably need to tune your antenna or “set the SWR” before using it, for best results. We love using these versatile radios when traveling off-road with our friends, but for them to work properly, you’ll most likely have to give them a quick tune up before use, to get the best performance.

Why is it important to tune your GMRS radio to match your antenna? Because if you don’t, any mismatch that occurs from having the wrong length of antenna could reduce performance, or even worse, damage your radio.

GMRS Antenna Basics: What Is SWR?

SWR stands for “standing wave ratio”, which is a measurement of how much RF (radio frequency) power is reflected back into your radio from the antenna vs. going in the direction you want it to go, which is outward. Any RF that’s reflected back into your GMRS radio is that much less power that you’re transmitting, and it may be harming your radio’s final amplifier.

On the chart below you can see just how much power is reduced at each SWR level.

swr chart — Chart showing how SWR affects power output.

surecom sw102 swr meter — The Surecom SW-102 comes with the connections most GMRS radios and antennas use.

Easiest Way To Set GMRS Radio SWR

To set the SWR on your GMRS radio, you’ll need the following tools:

SWR Meter. (We like “no set” models, which don’t require tuning, such as the Fumei RS-40* and the Surecom SW-102. (The Fumei RS-40 has the connectors that most GMRS mobiles use.) Also note: A CB SWR meter won’t work on your GMRS radio as they are for the wrong frequency range.
Small Allen wrench
Bolt cutters
UHF male Pl-259 to UHF Male Pl-259 jumper cable. (This connects the meter to the radio.)

Step 1. Of Setting GMRS Radio SWR

Move your vehicle to an outdoor location, away from obstacles.
Tune your radio to GMRS channel 16.
Set your transmit power to “low”.
Connect the meter, with your radio’s antenna connected to the “ANT” side, and with a jumper from the “TX” side to your radio. Set the “range” switch to the correct wattage.
Check to see if the channel is clear, give your call sign, along with “testing”, then key your mic and note the reading on the meter. If it’s 1.5/1 or lower, you’re good to go. If it’s higher, you’ll need to go on to step 2.
Carefully note the exact reading before proceeding.

swr meter gmrs — This reading, of just over 1.5/1, is a bit high. Note: set the range to match your radio’s power output on “low”, not medium, as we did.

Step 2. Of Setting GMRS Radio SWR

set swr anli antena — To set SWR, adjust the lowest part of the antenna. **Note**: We don’t recommend antenna placement next to the cab, as this affects SWR and reduces signal.

Mark the location where your antenna fits inside the base with a black marker, so you have a reference point for the steps below.
Start by loosening the screws that hold the antenna in place, then extend the length by pulling it up as far as you can go and still be able to tighten the screws.
Go back to your radio, key the mic, and note the reading. If the SWR is now higher, your antenna needs to be shorter.
If the reading is lower, tighten down the screws and call it good – if the reading is no higher than 2.0/1. (If it is, you can’t extend your antenna any further, and may need one that you can.)
If the reading is higher, move the antenna lower, below the black mark you made. Check the SWR again. If it’s lower, you’re on the right track. If it’s below 1.5/1, call it good here.
If your SWR is still above 1.5/1, you’ll need to cut some length off of your antenna. Proceed to step 3.

Step 3: Cutting Your Stainless Steel Antenna

Place the length of antenna in a vise and clamp it down. (You can get by without a vise, but one does help.)
Using a pair of bolt cutters, cut no more than 1/2 cm, or 13/64″ off.
Place the antenna back into the base – as far down as it will go – tighten the screws and check the SWR again. If the reading is below 1/5:1 you’re good to go. If not, you may have to repeat this process a couple times.

Important Note: Once you shorten your antenna you can’t undo it. Be sure that you’re “heading in the right direction” in terms of SWR before doing any cutting. Obviously the lowest SWR you can get is best, but don’t worry about it too much, since you won’t notice much difference in performance with SWR lower than 1.5/1

good swr reading gmrs — A good SWR reading of about 1.3/1. We can live with this, and stopped adjusting here.

That’s It, You’re Done!

Once you’re got your GMRS radio and antenna matched, and SWR below 1.5/1, you’re good to go. Make sure you tighten all the set screws and remove the meter. You can leave most SWR meters “in-line” but you’ll lose a bit of power output by doing so.

Note: If you still have any reading over 4.0/1, you may have a short in your antenna cable or other issue. Try replacing the antenna and cable to see it this corrects the problem.

If you’re new to GMRS radios, please check out our article “Choosing The Best Fixed Mount 2-Way For Overland Vehicles“.

Disclaimer: The above method of setting SWR works for me. Each setup is different however, so be sure to check your GMRS radio and antenna manufacturers installation instructions first for tuning instructions. Also, you may find affiliate links in this article to products we’ve used.

Full Disclosure: This post may include affiliate links. There’s no extra charge to our readers for using these.