Antenna Tuning Help

[rescuelt]

Hello,
I need to install some low band whip antennas on our vehicles, but I am not to sure about tuning them.

I understand the purpose of tuning an antenna, but I just am not sure how to do it. I have a Telewave 44A Watt Meter and I would like some help on how to read and operate it. If it makes any difference, I am using Moto CDMs at 60 watts, on 37.340.

If anyone can walk me through this it would be much appreciated!

Thanks!

[Tom in D.C.]

Basic steps:
(First, set the wattmeter's range switch to 150, then...)
1. Unplug the antenna from the transmitter and plug it into the output side of the meter.
2. Connect a short coax jumper from the transmitter to the input side of the meter.
3. With the switch on the meter in the FWD position, key the transmitter and read the output. You should get a relatively high reading.
4. With the switch on the meter in the REV position, key the transmitter and read the output. You should get a much lower reading. If you change the range switch to a lower range you'll be able to get a more accurate reading in the REV test position.

If the antenna is tuned right, the REV reading will be something on the order of 0 to 5 watts.
If the antenna is not tuned right the REV reading will be much higher, even approaching the
level of the FWD reading, which would not be good news. The longer the antenna the lower
the resonant frequency. Likewise the shorter it is the higher the resonant frequency. For
testing purposes, if you can set one channel in the radio to a much higher frequency then you
can run a test to see if the antenna is too short or too long to start with. If, say, you start at
37.34 and read 5 watts in REV, then switch to 42.00 and read 1 watt in REV then your antenna is too short for the 37.34 channel. Some lowband mobile antennas have screw adjustments but most don't; you have to start with a too-long antenna and cut it in SHORT increments of about 1/4 of an inch until you get a decent REV (which means reverse power) reading.

This should be enough to get you cranked and rolling.

Regards,

[akardam]

Well, first off, what kind of mount are you going to be using? Ball mount, NMO mount, or other?

If you're using a ball mount, you can buy a frequency-specific cut 1/4 wave rod, or just get a CB whip and cut it down (I did this for my truck). If you're using one frequency, as you've indicated, tune for that one frequency. If you're going to be using several frequencies, split the difference between lowest and highest and tune for that (but be aware that you're probably not going to get acceptable performance over more than 5MHz or so). Calculate the quarter wavelength in inches, and cut the whip a few inches long, and start trimming it down and testing reflected power with your wattmeter. When you get to something acceptable, there you go.

If you're going to be using an NMO mount, I'd highly recommend the Radiall Larsen base loaded lowband whips. They come with tuning charts for specific frequencies throughout their range. The same method as above applies if you're tuning for a specific frequency - cut long and trim down from there, testing all the while.

If you're planning on using something else, elighten us, and I'm sure someone will chime in with suggestions.

[rescuelt]

I am using ball mounts, and we have full length 96" 34-50 MHZ whips for them. The frequency info I listed was actually wrong. The 37.340 is the lowest frequency we will be transmitting on, it is the talk around channel. The highest frequency we will transmit on is 42.340, it is the repeater channel.

Thanks again.

[Tom in D.C.]

Quarter wave for 37.34 mHz is 75.2 inches.
Quarter wave for 42.34 mHz is 66.32 inches.
Midpoint for the two frequencies is about 70&3/4 inches.
But that's just theory, not real world.

I would start with the full-length antenna and see what
it reads in SWR. The mounting point, vehicle configuration,
etc. all contribute to what you end up with. The SWR
should be high, so then you can start cutting. The midpoint
of your highest and lowest channels is 39.84 mHz so if you
could set up a "tuning channel" and cut the antenna for
that frequency you'd probably end up with the best of all
possible worlds. Of course you'd remove the 39.84 from
the radio after you're finished.

[tvsjr]

The proper way is using a true antenna analyzer. I use a Tektronix NetTek myself - the Anritsu Sitemaster is another option, but it has some serious limitations. The (urgh) Mighty Fine Junk MFJ269 is supposed to be decent, but I've never used one.

[sethcwilliams]

The proper way is using a true antenna analyzer. I use a Tektronix NetTek myself - the Anritsu Sitemaster is another option, but it has some serious limitations. The (urgh) Mighty Fine Junk MFJ269 is supposed to be decent, but I've never used one.

You're absolutely correct, too bad most are too cost prohibitive for people who don't do fixed site installations professionaly. Their great if you build your own cables, you can actually see incresed loss across a connector and judge your handy work.

Quarter wave for 37.34 mHz is 75.2 inches.
Quarter wave for 42.34 mHz is 66.32 inches.
Midpoint for the two frequencies is about 70&3/4 inches.
But that's just theory, not real world.

I would start with the full-length antenna and see what
it reads in SWR. The mounting point, vehicle configuration,
etc. all contribute to what you end up with. The SWR
should be high, so then you can start cutting. The midpoint
of your highest and lowest channels is 39.84 mHz so if you
could set up a "tuning channel" and cut the antenna for
that frequency you'd probably end up with the best of all
possible worlds. Of course you'd remove the 39.84 from
the radio after you're finished.

rescuelt, just an FYI. When he says SWR, he's referring to standing wave ratio. You can get it by adding (FWD + REV power), subtracting (FWD - REV power), then dividing the former by the latter. (FWD + REV) divided by (FWD - REV).

Example:
If you have 10 watts forward power and 1 watt reverse power, add the two together giving you 11. Subtract the two giving you 9. At any point in the feedline, the forward and reverse power may be complimenting (increasing) or offsetting (decreasing) each other depending on the phase of the incident power to the phase of the reflected power. These two numbers represent those situations, your MAX voltage and your MIN voltage. If you follow the formula:

10w + 1w = 11
10w - 1w = 9
11w / 9w = 1.22

This gives you a VSWR of 1.22:1, meaning that your MAX voltage is 1.22 times your MIN voltage. The closer the ratio is to 1.0:1, the better matched your entire feedline/antenna circuit is and the more efficient your antenna system is at absorbing the power fed to it. All that said, it means your effective transmitter power (ETP) is as close to maximum as possible.

//ADDITION// For short feedlines like vehicle installations, this usually isn't a big deal but if you're doing a fixed site install with a feedline of say 100', it's usually a good idea (if practical to do so) to get a measurement of FWD and REV at the transmitter AND the antenna. This will give you some idea of the loss of your feedline. If your operating freq is HF (say 3MHz), not really a huge deal, but if you're installing a VHF or better system those shorter wavelength signals get chewed up pretty quickly over 100' of coax. //ADDITION//