Antenna Tuning

Posted: 11/29/2011 in Antenna Tuning
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Overview

SWR stands for standing wave ratio. Basically it is a very simple concept. When you transmit a certain amount of the wattage is reflected back into the transceiver. Damage to the radio will occur If the amount of wattage that is being reflected back into the transceiver is to high.

So the SWR is a measurement of how much signal is going out the antenna, versus how much signal is reflected back into the transceiver. An SWR level of a 3.0 will damage the transmitter. A 2.0 SWR is about an 11% loss and the transmitter of the radio will run hotter as compared to a lower SWR. Amplifiers do not like high SWR’s. A radio with a built-in amplifier should not normally be used if the SWR is above a 2.0. While they might get away with it the duty cycle on transmit is reduced. The transmitter will warm up much quicker with a higher SWR as compared to a lower SWR.

Let’s talk about setting the SWR’s

All CB antennas are tunable. Antennas that have an easy, often mechanical, tuning device are referred to as a “tunable antenna”. Accordingly, many people think the other antennas are not tunable and so they make no attempt to have the antenna tuned. This is a critical mistake.

Wire-wound antennas (without mechanical tuners), that test electrically long (higher SWR on ch 40), can be tuned by removing wire from the top. Remove the antenna cap, remove one wrap of wire and cut it off. Put the cap back on and recheck the SWR again on ch 40 & ch 1. As long as the SWR is higher on channel 40 continue to cut. Do not cut more than one wrap at a time. If SWR testing indicates the antenna is electrically short (higher SWR on ch 1), test for a ground problem. Touch the antenna bracket while keyed and watch for SWR fluctuation. Do not touch or even get close to the antenna. Move the coax around bending it, again watching for SWR fluctuation. If the SWR fluctuates a ground problem exists and must be corrected before proceeding. If there was no fluctuation bend the end of the wire straight up the fiberglass shaft and lap it over the very top of the fiberglass shaft. Check the SWR on ch 40 & ch 1. The goal is to reverse the situation so that ch 40 has a higher SWR than ch 1. If ch 40 is higher cut 1/8 inch of wire, put the cap back on and check the SWR on ch 40 & ch 1. Continue with this process until ch 40 & ch 1 are relatively the same.

Wire-wound antennas (with mechanical tuners), that test electrically long (higher SWR on ch 40), can be tuned by adjusting the mechanical device at the top of the antenna. Remove the antenna cap, and adjust the rod down or if threaded turn the device clockwise. Put the cap back on and recheck the SWR again on ch 40 & ch 1. As long as the SWR is higher on channel 40 continue to adjust the rod down or if threaded turn the device clockwise. If SWR testing indicates the antenna is electrically short (higher SWR on ch 1), test for a ground problem. Touch the antenna bracket while keyed and watch for SWR fluctuation. Do not touch or even get close to the antenna. Move the coax around bending it, again watching for SWR fluctuation. If the SWR fluctuates a ground problem exists and must be corrected before proceeding. If there was no fluctuation adjust the rod up or if threaded turn the device counter-clockwise. Check the SWR on ch 40 & ch 1. Continue with this process until ch 40 & ch 1 are relatively the same.

Solid fiberglass antennas (straight or helical wire impregnated in fiberglass resins) that are electrically too long (higher SWR on ch 40) can be cut shorter with a hacksaw or grinder. Cut only 1/4 of on inch at the most at one time. If the SWR test on one of these antennas indicates that it is electrically short (higher SWR on ch 1), the addition of a spring or quick disconnect (or both) is the only way to correct for the short condition.

Base loaded & Center loaded antennas with steel whips have a set screw (or two) just below the area that the whip is inserted into the loading coil. By loosening up the set screw(s) you are able to slide the whip up or down as required. If the SWR is higher on ch 40 slide the whip down, If the SWR is higher on ch 1 raise the whip.

Antenna Tuning when using an amplifier

Setting the antenna SWR with an amplifier can get involved. The antenna SWR is calibrated at the back of the amplifier with the amplifier in line and turned off. Once the antenna is properly tuned the meter is removed and re-installed at the radio with the amplifier in line. The antenna SWR level is then measured with the amp off and then with the amplifier switched on. The SWR level must be at an acceptable level at the radio under all conditions. The conditions are with the amp off on channel 40 and 1 and with the amp turned on, on all power levels of the amplifier on channel 1 and 40. Most shops don’t check the SWR match at the radio with the amplifier turned on. This is a critical mistake. With these steps added the SWR calibration can turn into a multi hour event at times.

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Comments
  1. Richard Earls, Harris Corp, USAF Retired says:

    This is very good info, I am an instructor with Harris RD Communication an retired from the USAF in Radio Maint. Keep in contact I like to ref your web sit from time to time to illistrate to students how this works in the real world.

    • htmcnetwork says:

      Hello Richard, I’m glad you like the Blog. I am the lead tech and head of R&D. The next step is adding video. This will be implemented in the near future. I am frequently adding information that will help educate my customers.

      Best Regards,

      Clay Thompson

  2. blaine hartley says:

    i was reading your section about checking SWR with an amplifier in line and have become a little confused. you instruct to remove the meter from back of amp and place at back of radio. if done correctly this will take the amp out of line as you have your jumper going directly from meter to radio and the antenna connected directly to meter. how will the amp be activated or “keyed” with no input? it will power on but no input will be sent to the amp if the meter is directly connected at the connector on the radio. i have always connected the meter behind the amp and checked SWR with the amp turned off. once i have acceptable numbers i then turn the amp on and re-check. it might also be a good thing to note that most radios and amp setups are at somewhat of an imbalance. the carrier of the radio is often too high for the amp causing a “harmonic” issue which will cause inaccurate SWR readings. this may be “over the heads” of the basic user. but should be mentioned if they plan on purchasing an amp. the radio and amp should be “matched” together for optimum performance. not trying to correct anyone as i am no expert by any means but i just got confused when you mentioned removing the meter from the amp and connecting directly to the radio. mabee i need a drawing since the words are not connecting in my head. sorry for stepping on anyone’s toes as this was not my intention.

    • htmcnetwork says:

      Maybe I should expand on the procedure a bit. When I stated to remove the SWR meter from the back of the amp and install it at the back of the radio I mean to reinstall the antenna coax to the back of the amp, then detach the coax at the radio and put the meter in line at that point. The amp will then be in line with the meter in line between the radio and amp.
      You must measure the SWR level at the radio with the amp off and on and at all power levels. If the amp has 4 power levels the SWR must be acceptable on all 4 levels at the radio and with the amp off.

      Regards,

      Clay Thompson

      • blaine hartley says:

        thanks for expanding that for me. i have never heard of doing this in the many years i have been around radios. i’m still trying to wrap my head around checking the swr at the radio between the amp? i was always under the assumption that you wanted to read the swr at the output point at maximum power level. could you explain the purpose of having the meter between the radio and amp? i’m eager to learn something new and this concept is of great interest to me. if there is something i can learn to improve my system i am always all ears. so what exactly am i reading with the meter in line before the amplifier? would this be a way of ensuring that there is no feedback to the radio causing a high swr? i always was under the impression or was “taught” that once you read the swr behind the amp with it turned off. then once you adjusted barefoot you really didn’t need to check with the amp on. i always did just to make sure but now that i am aware of this method i am curious as to the reason of the meter between the radio and amp? if the swr is ok behind the amp what would cause it to change (other than power increase) between the two? i guess i seem to be repeating the same question in different forms so i will get my learning goggles on and await your explanation. a close friend of mine ordered a radio from you several years ago and suggested i check out your site and i must say i am glad i came here!

      • htmcnetwork says:

        Most service departments wont attempt it. It’s not that difficult if you understand what we are doing. With the amp on you are actually measuring the input match of the amp not the antenna SWR. If the input match of the amp is high damage to the radio will result.

        Best Regards, Clay Thompson

        Clays Radio Shop & Motorcycle Communications Support 817-374-4675 | Sales 800-917-6698 Follow us on twitter or Like us on FaceBook Biker Blog – http://htmcnetwork.wordpress.com/ Auto Blog – https://htmcnetworkcitizensband.wordpress.com/2011/02/20/hello-world/ http://www.claysradioshop.com | http://www.motorcycle-communications.com 2280 West FM 140 Pearsall, TX 78061

      • htmcnetwork says:

        One additional point, you can’t accurately check the SWR at the antenna side of the amp with it on. Spurious harmonic content will degrade your results.

        Best Regards, Clay Thompson

        Clays Radio Shop & Motorcycle Communications Support 817-374-4675 | Sales 800-917-6698 Follow us on twitter or Like us on FaceBook Biker Blog – http://htmcnetwork.wordpress.com/ Auto Blog – https://htmcnetworkcitizensband.wordpress.com/2011/02/20/hello-world/ http://www.claysradioshop.com | http://www.motorcycle-communications.com 2280 West FM 140 Pearsall, TX 78061

  3. blaine hartley says:

    thanks a lot for the explanation. i can see the light now. i was aware of the harmonics that are created by amps so i really didn’t check very often at the output with the amp on. i would just monkey around to get a somewhat ball park estimate of things. but now that i know the input match is of more concern than anything after the amp i will be doing a little testing to see what is really going on when i mash the gas so to speak. thanks a lot for the education. i am partial to my current radio technician but when the time comes for the next purchase i will try my luck with one of your radios. i have heard many of them on the air and they all sound great. thank you again for sharing the knowledge.

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