Anyone had this problem?

You are suppose to check the swr with the weather cap on. Should tell you that in the directions that came with the antenna.

 

You adjust the tip, put the cap back on, and check the swr. If it needs further adjustment to get 1 and 40 the same, then remove the cap and adjust up or down depending on which is lower, 1 or 40, and put the cap back on and check swr. Continue doing this until you get channel 1 and 40 at the same swr.

 

Just a thought (as I so seldom have any....)

Are you closing the door(s) after each adjustment, and transmitting from inside the truck? Or are you (as would seem more logical and *way* easier) standing on the step with the door open, fiddling with the tip, then leaning inside to check the meter?

It's easy to see why a person would do it the second way, because it's way more convenient than climbing back inside, closing the door, and then keying the radio and reading the SWR.

Except that the SWR is likely to be different when the vehicle is out of its normal operating mode, which would be with the door(s) closed. Having a door open affects the "shape" and sheer amount of the antenna's ground plane. Also, if your mount doesn't move with the door, then the door is actually closer to the antenna when it's open.

And, while it doesn't make sense at first, the tip does affect the tuning.

It would be better to tune the antennas individually, with a piece of regular coax (not the co-phased harness). Once they're both tuned separately, *then* hook up the co-phasing harness to the antennas and radio. As long as either meter shows an SWR of less than around 2.0:1, you've probably got a good match. It won't be perfect, because the co-phasing harness is working as an "impedance transformer" by having a bunch of little mismatches along the parts of the antenna system. And it *is* one of those times when the length of the coax really does matter, because the whole system is not a perfect 50 ohm impedance. But your radio will be happy with it.

But even at 2.0:1 SWR, you're only losing about one-tenth of the signal (actually 11&#37 into and out of your radio, which is not enough to make any change on anyone's S-meter (including your own). A stock 4-watt AM radio looking at a 2.0:1 SWR still delivers 3.56 watts to the antenna. The difference at the other guy's radio would be less than a needle's width on his S-meter.

It's good that you're using an outboard SWR meter instead of the one built in to the radio. The ones installed in radios are usually enough to tell if something's drastically wrong, but not good enough for lab-grade measurements. But both that meter and the outboard should at least show when your reflected power is lower or higher than each earlier try. If you think of them as a relative indication, rather than an absolute and correct value, it may help alleviate some of the common angst with installing. Just tune for minimum reflected signal, then write down what it was on either meter (or both) so you have the reading for reference. That way, if something changes, you'll have the original reading to compare it too.

If either meter reads above 3.0:1 ("3 to 1") when you transmit, it's time to go looking for whatever has failed. Usually it'll be either a short to ground (often happens with a broken nylon insulator at the stud mount), or a broken path to the antenna (typically from the center coax wire coming out of the plug, or a broken antenna).

So, tune the antennas separately with regular coax and the doors closed and the tips in place. Then hook up the co-phasing coax to the antennas and radio. If the SWR is less than around "2 to 1", you should be golden. Any minute differences in SWR from there will hardly be measurable, and will never be heard by you or the folks you're talking to.

Hope that helps,
-- Handlebar --

 

I'm glad you're buying a harness. The temptation when building one's own is to use regular 50-ohm coax. The coax necessary for making a harness to split a single connector into two separate 50-ohm antennas is actually 72- or 75-ohm line of a specific length. If a longer feedline is needed, then regular 50-ohm coax can be put onto the radio end of the harness, and that run to the radio.

As for the difference in tuning, I'd expect some difference between single and dual antennas, since the "antenna end" of the harness probably isn't exactly at 50 ohms, so the antenna would need some adjustment to match its feedpoint impedance to match the coax by adding some electrical length.

Either that or something else. <----- Handlebar's Universal Disclaimer&#174; (TM Handlebar Enterprises Int'l, 1981)

 

co-phasing antennas is a waste of time if you do not have the required distance of 1/2 a wave then it is pointless and you will gain nothing (literally) for the radio. the entire purpose of running cophased antenna is to get more signal gain. but if you do not take time to read on the physics, you are just wasting time and money. here, have a quick read.....

http://www.signalengineering.com/ultimate/co_phasing.html

 

http://www.signalengineering.com/ultimate/coax_basics.html

 

Aww you gotta have two antennas so you can tilt them forward and look like a ###### going down the road LOL