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dipole on a big rig swrs

Yes , i agree i had a similar set up only both antenna were isolated from ground.
It worked ok but as you say not as well as the single grounded antenna.

While I can't quote theory, I do know what works in trucks

That is the key. Theory has to be put aside , if it works go with it.
 
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With 2 loaded antennas matched for 50 ohms and even using 50 ohms coax, I would expect your feed-point impedance to be even higher around 100 ohms. This can be verified easily if you have an antenna analyzer one or know a real CB shop that has one.

This is a easy fix using a 2:1 balun. A balun is a transformer device used to balance an unbalanced feed-line for feeding a balanced antenna, Hence the name BAL-UN. At a 2:1 ratio, this means it will cut the feed-point impedance down at 100 ohms in half to 50 ohms which should give you a nice flat SWR match. 50 ohm coax cable is an unbalanced feed-line.

fourstring,
I have made several 11 meter dipoles from Firestiks that seemed to work pretty well. I don't have an analyzer so I don't know their impedance. Is 100 ohms harmful to the radio? Will reducing it to 50 make a big difference in performance? I don't know how concerned I should be about it.

Thanks.
 
100 Ohms isn't going to be ideal. You won't get a big or even noticable difference in performance getting it down to 50 Ohms but the finals in your radio/linear will thank you for it.

Thank you, M0GVZ.
I've often wondered about that.
 
fourstring,
I have made several 11 meter dipoles from Firestiks that seemed to work pretty well. I don't have an analyzer so I don't know their impedance. Is 100 ohms harmful to the radio? Will reducing it to 50 make a big difference in performance? I don't know how concerned I should be about it.

Thanks.
If the mobile antenna system is measuring 100 ohms at the antenna feedpoint, then an analyzer should show a SWR at 2.1. That's why I asked the question and the responses say the impedances stay at 50 ohms using 50 ohm antennas and coax measured with an analyzer.

A 2.1 swr represents 11% wasted reflected power. Couple that other losses inherent to mobile antennas and the losses can add up, but as MOGVZ said, the real affect is with your radio and not so much in losses in the other end of another stations receiver.
 
A few things.

I have tuned a set up kind of like this for someone in the past, two firestik antennas in a center fed vertical setup on a large fiberglass vehicle on a mirror mount (in my case a motor home). The feed point impedance at resonance for me was just under 75 ohms. I had to use a choke on the antenna as it had serious CMC issues, so if you know how to check for said issues you should. A pair of vertical antennas in a center fed design with one of the antennas tips that close to the ground will never actually be balanced. The ground will affect the lower side more than the upper. It takes quite a bit of height before that effect lessens to the point of irrelevance, far higher than any mobile install will ever be.

If the antenna has a feed point impedance higher than you are comfortable with, you can use a 1/4 electrical wavelength of 75 ohm coax between the 50 ohm coax and the antenna to convert the impedance. The conversion isn't perfect, 112 ohms or so, but it is much better than the potential 2:1 or higher SWR you would get without it. The thing about this is, you really want to talk to someone who can make a custom 1/4 wavelength 75 ohm coax for you as their bandwidth is limited to begin with, and have it made with your center frequency in mind. It isn't a bad route, but it will narrow the apparent bandwidth that you would have gotten without needing such a device. That being said, other methods to do the same thing will also narrow said bandwidth, and likely be more lossy as well.

Not all power reflected because of an "imperfect" SWR is "lost". Many charts list said SWR to loss figures, but there is a difference between being "reflected" which is what is really happening, and being "lost". If the circuitry in the radio is designed properly, all of the reflected power that makes it back to the radio will be "re-reflected" back towards the antenna. The actual loss happens as this reflected, and later re-reflected signal, travels banc and forth over the coax. This re-reflection along with a high SWR is what many CB shops do to trick their power meters to show a higher power output. Treat said figures as an overestimation of the problem, an easy number to find and use, but not completely accurate. In reality, how much loss you get from SWR depends on the quality and length of the feed line used. While mobile installations generally use low quality coax, it is also, luckily, very short. In any case, even at 11%, that is not nearly enough of a difference that you or the people you talk to will actually notice.


The DB
 
I had to use a choke on the antenna as it had serious CMC issue
I had the same issue with CMC when I tried it years ago on my mobile to where it wasn't worth trying to surpress it especially when running power.

I didn't have analyzer at the time but just used 2 firesticks and tuned them with a cheap SWR meter to just under 1.9.

My guess at the time that even with 50 ohms firesticks and coax, the antenna system would stay around 50 ohms throughout, turns out that's not the case. This was based off the folklore from people who have done this claiming excellent results.

I asked 2 forum members who claim they checked these with an analyzer and they said they saw near 50 ohms. One was an attic antenna, the other was mobile.

I'm still reluctant to believe that the feedpoint impedance would be near 50 ohms but rather more close to 100. You measured around 75 ohms, that I'll believe.
 
I've often heard it said that impedance is reduced by arranging the legs of a horizontal or vertical dipole into a "V" configuration. If oriented at a 90 degree angle, how much of a change is typically noticed?
 
Riverman71, here is a good link that explains a lot about VSWR and impedance matching. Actually has a tool at the bottom for calculations.
http://www.cdt21.com/resources/TechnicalTools/vswr1.asp
Hope this helps a little.

Thank you, 222.
That site is full of great info! However I was unable to find a way to compare the differences in impedance between a horizontal dipole and an inverted V dipole.

If a horizontal has an impedance of 75 ohms, what can one expect after changing it to a 90 degree inverted V? 65? 55? 35? Am guessing lots of guys have experimented with the two types and am curious what they found.

Thanks!
 
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Thank you, 222.
That site is full of great info! However I was unable to find a way to compare the differences in impedance between a horizontal dipole and an inverted V dipole.

If a horizontal has an impedance of 75 ohms, what can one expect after changing it to a 90 degree inverted V? 65? 55? 35? Am guessing lots of guys have experimented with the two types and am curious what they found.

Thanks!

71, here is my model of an old setup on a big truck back in the days when CB was a big deal. Their setups used shortened elements, but they worked great and these two guys (Ted and Bill the owners of the Sky King Antenna company) could talk all over the Nation it seemed...back in those days.

The model results will show a match of (43.46 + J 0.8695 for an SWR = 1.152) in the source data report. IMO the gain as shown in the Pattern Report is also exceptional compared to a whip in the middle of the truck on top and rear of the cab.

Based on my model, I hope this helps with some of your questions.
 

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Here is the same model with the antenna mounted on top rear of the cab.

Notice that I left the bottom element in the 1/2 wave model in this model to show I just moved the radiator over to the cab. The adjustment for the impedance is affected by the angle.

Note as I tuned the match for this model mounted on the cab I had to shorten the radiator some.
 

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Thanks, Marconi.
But that's not exactly what I'm looking for. Those appear to be "L" dipoles with a 90 degree angle. I'm wondering about an inverted V dipole with the coax hanging straight down being compared to horizontal dipole and wondering what the difference in impedance typically is with all other things (environment) being equal.

I would assume some of the differences in your models are due to the location of the antennas being different. And shortening one of the radiators is another fly in the ointment when comparing them isn't it? I don't know, just asking.

Anyway, this is not the proper thread for my question. The antennas I have in mind are not attached to a vehicle. I know I sound repetitive but I can't help but think those with dipole experience have a idea how these two types generally compare as far as impedance is concerned. I've fooled with several myself but don't have an analyzer. If I did I wouldn't be asking this question.

Again, my apologies for this question not being in the proper thread. I really thought I'd get a quick answer. If I need to post it in a different location, please let me know.

Thanks, everybody.
 

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