Batboard

For you professionals out there.......Do you use an swr meter when installing an antenna or just cut the whip to length like our local installers do. I am an amateur radio operator and was moving a radio to my Chiefs new truck last night. I couldn't believe the SWR reading I got on his antenna. I then checked out one of the fire trucks and the SWR was 3:1 !

What is the standard for these installs ?

Thanks
Kevin

Kevin:

3:1 is not acceptable. Like you, I'm a ham, and I use a Bird 43 wattmeter with the appropriate slug for power and band when I set up a mobile, of which I've done many.

The last one I did was in June when I got a Subaru Outback wagon. I had an NMO mount installed, then I put a dualband whip on it. The 147.00 mHz reading was about 1.2:1 and the 446.00 mHz reading was a shade higher. (The radio is an FT-90R.) All this without trimming anything on the antenna, which I probably could not have done anyway since it's a dualband antenna which cost all of $14 from Antenna World in Florida.

I don't trust the cheap SWR meters at VHF and UHF frequencies. A good Bird 43 will cost you about $150/$175 if you shop around, and a proper slug another $40/$50.

Tom, W2NJS
...in D.C.

ouch, that hurts to see how high the swr is, how many watts is that FD radio putting out?

The one in our chiefs truck is a 90 watt lowband (46.060 Mhz ) GE unit (not sure on the model). I think that tonights project is to check and tune the antennas on all the trucks. The reason that I got the job of switching over his radio is because he likes my workmanship better than that of the company that has the county contract for radio installs. After what I saw tearing everything out of his old truck I wouldn't let them in mine either !

73

Kevin

I haven't seen one radio dealer set SWR yet. They just cut n' stuff!
I use a cheapie MFJ-269 with the UHF option. It was only $250 and supplies the low power rf signal for you. I checked it against the Bird and it's right on. Great unit for field installs.
Basd if you ever need service or parts since MFJ won't even talk to you unless you want to buy a new one.

I know lots of guys who do that.

Make shure to pull the whip out and see if it goes up or down, you should get some more Whips just incase they cut them short.

Plus thos cuttin charts are no good 9 times otu of 10

i used comtelco high band with spring chart is close but needs some fine tunning.

If they are all the same brand antennas tune one then cut the other to just a little longer and then fine tune

Being an installer at a MRSS we ALWAYS (I repeat ALWAYS) tune antennas. Our most used antenna is Spectrum, which the tuning chart for is pretty accurate. If it is for a customer we always deal with, once we find the right length we note it and always cut to the same length. If it is for a customer that we have not dealt with, we cut 1" less than the chart says and fine tune. Most of our work is for government so they use wide band antennas that normally come out pretty good. With such a high SWR did you possibly check to see if the connector was shorted from the body to the tip. This will give you a high SWR. By the way, we use telewave wattmeters, which personally I like better than Bird for the simple fact you don't have to mess with slugs.

P.S. If the connector is not shorted, a poor ground on the antenna will do the same thing.

When I did the install I used a new piece of coax and PL-259. I did make sure that there were no shorts and the mount is grounded to the fender. I am just going to have to go through the fleet and check them all out. Thank you for the replies !

I have found that low band swr is often tricky. When I check the units, i make sure they are parked away from other vehicles and all doors are closed. The MFJ is a great tool for this as the swr sometimes goes in peaks and valleys (making it easy to cut too short).

I note from the N2 call that you're in the Northeast.

With the classic "whip into the socket & clamp with setscrew", there
is a constant problem with corrosion from road salt, etc.

I find that coating the setscrew hole and whip socket with "antiseize"
(lubricant with metallic flakes used to prevent aluminum from seizing to steel) before installing the whip & setscrew works well.

I also put some on the stud (if any). The anti-seize is a very thick oil/grease & effectively prevents any water from getting in to cause corrosion.

Silicone grease around the base washer of the NMO and on the brass threads of the NMO before screwing on the cap.

It solved my high-SWR problems for good.

Poor ground plane area is one of the overlooked problems on lowband antennas and their instalation.
Even with a good ol B1rd thruline meter, you can get errors on lowband and wind up with the rod cut too darned short, I've been up that road several times.
Has anyone tried the Autek Reasearch UHF antenna meter? I was thinking about getting one.

It's probably better not to tune a LB antenna inside a firehouse with lots of relective things around.... like big fire engines, squads, metal walls etc..

SG

I sent an MFJ 249 in for service with no problems. This was about a year ago. They had a fairly quick turn around time too.

mancow

on the topic of lowband trickiness..... my ambulance agency just got a new ambulance.....and the radio company mounted the antenna on the front hood right in front of the driver (i.e. exactly across from the FM radio antenna). The reason? - because when mounted on the roof it may hit one of the bridges we sometimes drive under. The result? Depending on location and position of the rig - performance varies. Not to mention the driver getting 110 lowband VHF radiation to the face.