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Best shortest mobile CB antenna??
- Thread starter Galaxy 959
- Start date
What would be a good mount for this spring and 102 whip? I could go stake pocket or drill a hole in top of bed rail or put a mount right to side of bed.
http://www.ebay.com/itm/Antenna-whi...hash=item5883dbc49f:m:mjD9aCVMFQewgrF7rbMZ3NQ
I could find one of these in stainless and put it on the bed rail.
I could find one of these in stainless and put it on the bed rail.
Top.
Better to have the sheet metal of the vehicle below the antenna, rather than beside the sheet metal.
If it is mounted to a rail or side; then it becomes directional. Skews its transmit/receive polar pattern.
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if you are gonna have a 102 inch whip mount it in the middle of the bed right behind the cab.. just like in the picture.
I can't speak to the lower bands offhand as most of the modeling I do tends to be used on the CB forums so I tend to use CB antennas. I have seen examples where a cap hat was used in conjunction with a coil just under it, but I also got the distinct impression that that was for tuning purposes more than anything. Perhaps at some point I will model a low band antenna to confirm that. Anyway, here on the CB band, there is no real difference between a top load and a cap hat, and for that matter a base load as well.
Take a loot at the radiation efficiency (Radiat-eff. number) on these three antennas...
Five foot antenna on Ford Explorer model, base loaded:
Five foot antenna on Ford Explorer model, top loaded:
Five foot antenna on Ford Explorer model, capacity hat:
Gain comparison...
The DB
Liked how your software showed the gain figures too.
So there is no doubt about a compromised gain figure.
No magic antennas here; just differences in that compromise.
So there is no doubt about a compromised gain figure.
No magic antennas here; just differences in that compromise.
Just one more bit of data, here is a comparison of the same three antennas 2.0 and lower SWR curves... Each side of the curve is capped with one SWR value that is over 2.0 to see the line crossing...
Top Load bandwidth, just under 0.5 MHz.
Base Load bandwidth, just under 1.5 MHz.
Cap Hat bandwidth, just under 2.5 MHz.
The DB
Top Load bandwidth, just under 0.5 MHz.
Base Load bandwidth, just under 1.5 MHz.
Cap Hat bandwidth, just under 2.5 MHz.
The DB
Certainly the top load loses to bandwidth, while the cap hap wins that hands down.
The base load looks typical.
Good to know . . .
The base load looks typical.
Good to know . . .
We are getting too cerebral here for a CB thread. But, a cap has is NOT used for tuning purposes. That's what a loading coil is for on shortened antennas. A cap hat on a whip with no coil won't tune properly. I tried this once for myself in the real world without resorting to computer modeling.
Also that modeling chart can't be correct since a 5 ft. antenna with a capacity hat will be on a different frequency and depending on how many capacity rods and their length ( You didn't consider that did you? ), it will probably be somewhere around the 15 meter band. YET YOUR MODELING PROGRAM SAY IT'S STILL ON THE 11 METER BAND.
SO YOUR MODEL PROGRAM IS WRONG!!!
I'm surprised Robb didn't catch it, but It shows he is out of his element.
The band width chart I can agree with since my real world experience with my current screwdriver antenna without a cap hat and a 4 ft. whip had a usable bandwidth on 20 meters of 2.0 SWR +/- 120 KHz centered on 14.225 MHz without retuning and with a caphat, I now cover the whole phone portion and more of the band without retuning. That's an increase of over 130 KHz bandwidth before retuning. All the upper bands above 20 meters with the cap hat now have full band coverage with the cap hat on 40 and 60 meters increased about 50 KHz.
I should note my original screwdriver antenna only tuned down to 40 meters, but with the cap hat, I now can tune 60 meters with some coil turns left over.
I suggest you do what I and most others do, apply your knowledge and experiments in the real world with hands on test equipment and see what works and don't rely so much on modeling programs.
I guess this is an odd forum then, here you are by far more likely to see this type of discussion on the CB antennas thread than the ham antennas thread. Go figure...
The combination of the cap hat and the top load coil were for tuning purposes, not just the cap hat. The use of a coil shortens an antenna, but it also lowers R at resonance. A cap hat shortens an antenna while raising R at resonance. Balance the size of the cap hat and loading coil and you can control the resonant frequency and where R ends up at resonance. Using either the top load or a cap hat of the appropriate size and you can still find resonance. To say that a cap hat cannot be used to tune a shorter than 1/4 wavelength antenna is just plain wrong, although most cap hat designs are not adjustable so this is by far the more difficult route.
Wow, do you really have that little faith in my modeling skills?
So about what you think I didn't consider, the length of the elements of the cap hat. (While not being addressed here I can make a cap hat of nearly any size and shape with modeling software) In my case there are four elements. One tip of each element is set at the same dimensional point as the tip of the vertical element of the antenna. The other tips points are adjusted by a variable and the direction of the elements are 90 degrees from the adjacent elements, and all four elements are on the same plane. I designed the model specifically to make tuning said cap hat easy, all I have to do to change the size of all four elements at once is change one variable. If reactance was negative I knew the antenna was electrically short, so I added length to the four cap hat elements. If said reactance was positive said elements were to long, and I adjusted accordingly. I made continuously smaller changes to the length of the cap hat until the antenna was resonant. It wasn't even hard to do, as a matter of fact, it was quicker and easier than tuning either of the coil based loads used in the other two models that one was compared to...
Seriously, have you read none of my posts on this forum in the past? Perhaps before you assume I would miss such a basic step again you should.
I have done far more experimentation than you seem to think, although in my case not directly with cap hats. I also have an AIM4170c VNA that I use when working with antennas, among some other equipment, including some older equipment that does some of the same things such as a noise bridge and a dip meter. I break out said equipment for fun sometimes. I am well versed with working with real world antennas and modeling, and seriously, in my experience they provide remarkably close results.
Here is a question for you, have you ever made a cap hat that you can change the size of to tune the antenna?
The DB