high swr with co-phased 102" steel whips

[Marconi]

That particular vehicle model has 203 wires. I think that should be sufficient for an object that size.

I don't have near as many wires, but I agree 203 wires should be enough.

 

[The DB]

I'm mostly done with a ford explorer object to model around. As this is getting a bit off-topic I'm going to create a new thread for it when I am done...


The DB

 

[hotrod]

I've always tuned one antenna at a time then hooked up both and fine tuned, I've seen where they were usually long before fine tuning so my experience agrees with this. However these numbers are larger than I would have expected even considering the size of the car, very interesting.

It doesn't seem intuitive that resonant lengths would change like this, is the reason for this happening known?


PS:



Be sure to make a model of a Suburban with a PVC pipe stuffed up its nose.

thats the way you should do it. and just to add to that use a single 50ohm piece of coax to start i used 18 feet. use that for each antenna. then i use a co phased harness with two coax leads both 75 ohms 18 feet. . no T connectors.

 

[hotrod]

Here are some results from the car model I used a while back. I included a 1/4 wavelength ground plane antenna with 4 radials at the same height as the car roof for reference...

I know, I called it collinear, but it was actually cophased as the picture below shows. That was an error on my part when I made the models a while back.

And a picture of the vehicle model I used...

Yes, I know it is a small car, but its what I found that I could make work at the time.

Resonant lengths for these antennas came out different for each of these models as well... For the 1/4 wavelength groundplane, the vertical and the radials all came out to 9 feet. The single antenna in the center of the roof came out to 7 feet and 8 inches. The cophased antenna lengths came out to be 6 feet and 10 inches. Note, these numbers are from being modeled over a very small car, smaller than most people use, most vehicles would require longer antennas to be resonant...

At some point I intend to make/find other vehicle models to use when modeling. Modeling mobile antennas is almost never actually done partially because of the difficulty of making said models...


The DB

thank you for posting. im not 100% sure at what. it is. radation pattern???

 

[The DB]

thank you for posting. im not 100% sure at what. it is. radation pattern???

Yes, its the radiation pattern.


The DB

 

[rabbiporkchop]

ok, im trying to figure out how to lower my swr in my mobile. i put in a brand new piece of 18 ft. coax, and i have mounted two 102" steel whips, one on each side of course, to the front of my pickup bed. i used a piece of 3/8" flat bar to run across the corner of the bed on each side. it makes for a very sturdy mount. anyways, the swr is a 2:1 bearfoot and drops to 1.5:1 when i turn my amplifier (300 watt Fat Boy). when i modulate with or without the amplifier on, the swr jumps almost off the meter. is this a grounding issue, or does anyone have any ideas about how to lower the swr on this particular setup? thanks.

"Kamikaze"

If you hook an analyzer up to one of the elements and sweep it to find a low spot you will notice that when you add the second element that both elements will be electrically too short once you add the second element.