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Best shortest mobile CB antenna??

Just going by all the antenna theory books I have read concerning mobile antennas. It's the main reason top loading is used in LF and VLF arrays and some AM broadcast antennas. To increase efficiency. I am 99.9% sure I am correct but will reserve that 0.1% for later when I get home from work and can double check my books. :)
 
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Captain Kilowatt said:
Just going by all the antenna theory books I have read concerning mobile antennas. It's the main reason top loading is used in LF and VLF arrays and some AM broadcast antennas. To increase efficiency. I am 99.9% sure I am correct but will reserve that 0.1% for later when I get home from work and can double check my books. :)
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Never mind the books as I can't read. But, IMHO the top loading antennas work for my typical install.
 
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The DB said:
The so called mobile CB 5/8 wavelength antennas are not actually electrical 5/8 wavelength antennas. An electrical 5/8 wavelength will have a very high X (reactance) at the feed point just like the full length 5/8 wavelength antennas. Because of this they would require additional matching just like the full length 5/8 wavelength base antennas which they do not have.
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I do believe that is what I said. No receive gain and capacitors and inductors would be needed. I believe that means antenna matching circuits. I always marvel at my radios automatic tuner and the wide spectrum of frequencies that it can make my antenna resonate to. My QSO King antenna has a 9:1 BALUN and 66 ft of wire hanging in the breeze and it works very well. Also if you notice "The Quotation Marks" that means I don't buy into that particular fallacy.
 
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Back to the question that the original post asked: is there an antenna that is efficient and short? Answer is no; as there are trade-offs. Is there an antenna that will pose less of a threat to getting knocked over and still work OK? Well if 'OK' is what you need, then there may be two that come to my mind that I've used with fair results.

One is the antenna that unit_399 mentioned, the 5' FirreStik. That will you put the height at/around 10 feet/overall. Another choice is the Wilson 1000 or 5000. The FireStik is a top load, and the Wilson is a bottom load. Like the Wilson over the Firestick - IMO.
 
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Road Squawker said:
Gee, I dunno 'bout that... once a loading coils is placed ABOVE about 1/2 way up a mobile antenna, the efficiency does decrease.

relocating the loading coil to the approximate center of the antenna, the current node will rise.. This fact doubles the radiation resistance. to this point, the efficiency has increased, however,..., but there is a bit more coil (Q) losses as center loading approximately doubles the required reactance for resonance.

locating the coil higher up (higher than center),will increase Q losses that will more than offset the increase in radiation resistance.

The resonant loading reactance varies inversely with the antenna length. the Efficiency varies slowly with load point and peaks at roughly 0.4 of the whip length.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=37&cad=rja&uact=8&ved=0ahUKEwiZmMKqj6PNAhUiGFIKHd7NAOw4HhAWCDwwBg&url=http://ieeexplore.ieee.org/iel5/25/34022/01622246.pdf?arnumber=1622246&usg=AFQjCNHVyBWhL0SIqgNY92ED-14y_y664g
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The ARRL Antenna Book series disagrees with this, as does modeling. Efficiency does not peak when the load is near 50% of the way up the antenna then drop off, instead it continues to climb as you move the load further up the antenna. Not that you will notice the difference with the load at the top or bottom though, the difference in radiation efficiency is less than 2% according to modeling, and my Ford Explorer modeling object. For the record, gain also continues to go up and the load is placed closer and closer to the tip in my models, although, the difference isn't as much as many people want to believe, less than 0.05 dB difference between the lowest gain model (the base load model) and the highest gain model (the top loaded model), and there is no peak in between where the linked article claims you should see one.

I didn't check the SWR bandwidth when I was testing this with modeling, so I have no comment on the Q of the antennas modeled. However, there is a rather large difference between Q and losses. Losses effect Q, but an antennas Q does not, in and of itself, cause any loss as your wording suggests.

The reactance required for the load not only varies based on the length of the antenna, but where on the antenna the load is. The further up the antenna the larger the coil you need to get an equivalent amount of shortening of the antenna. Someone already mentioned this above, although it was stated the other way around... ;)

And I'm sorry, but that article is full of errors. I can demonstrate several of said errors with modeling software if need be... The current peak, for instance, is not in or above the load. A loading coil will cause there to be more current immediately above it than if it weren't there, but this is not the high current point of the antenna.


The DB
 
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While some of you Hammies ;) are showing off your working antenna knowledge and debating among yourselves, you haven't given much advice on specific antennas or realistic home brew ideas. So I'll give mine.

I once made my own home brew mobile antenna for 40 meters using a 20 meter resonator and a 24 inch diameter capacity hat along with some feed-point inductance, had an excellent match and the antenna length was just under 4 ft.

I basically copied the Hustler design as shown in the pic below and used the DX engineering capacity hat system that I now currently use on my mobile screwdriver antenna.

hr-1p-example.jpg


With some ingenuity, antenna analyzer, and antenna knowledge, one can make a coil with 14 gauge enameled wire wrapped around a 1 1/2 in. PVC pipe with caps drilled out for antenna studs and a 22 in. mast. Just temporarily match it for 6 meters (or close to it) with a 4 or 5 ft. whip, then replace the whip with a capacity hat and add or subtract coil to get 50 ohms. Then they will end up with CB antenna with a low profile and more efficient over any typical commercially made 4, 5, or 6ft. CB antenna.

While a shortened mobile antenna is always less efficient over a full size 1/4 wave one, you can make a shortened mobile antenna more efficient than it's original design.
 
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Agreed however most people don't want an antenna nearly as wide as it is tall. LOL Capacity hats are great for improving the efficiency of short antennas and are a form of capacitive loading however most folks tend to shy away from them I think for esthetics (I think they are fine) or perceived mounting issues with the extra width and wind drag.
 
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StrangeBrew said:
If the back stake pocket is an option I would consider a 102" steel whip with a good spring and just let it hit what it wants.
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I was thinking the same thing. I found the ones from DX Engineering are much thicker, and take (or give) a beating better than the Radio Shack ones that are much thinner. And then he gets the best of both worlds. A really efficient and effective antenna that isn't a compromise. I have whacked all kinds of stuff with the DX Engineering version, and it still is not bent.

I have run 4' and 5' fiberglass (Wilson silverload and Fire Stick), and they all worked pretty good. Also ran a Lil Will and a K30, and they did well for what they are. Predator I10K, which works great, but really grabs people's attention. I'm now running the 102 inch whip. It blends surprisingly well into the background, too!


73,
Brett
 
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Hammy... Never heard an actual ham use that term...

Anyway...

Unless there is something wrong, for an antenna that is shorter than 1/4 wavelength in length there really isn't much you can do to increase how well it will perform. You can move the load towards the tip for a slight increase in efficiency. You can replace the coil based load with a cap hat for slightly better efficiency. Even still, the efficiency gains from doing this will be minimal at best, you won't notice the difference.

That being said, the best thing to do efficiency wise for a mobile antenna is get as far away from the earth below as possible. Height really does matter, and it matters even more when you are this close to the earth. The next thing to do is bond your vehicle, and I meal all of it. Bonding the entire vehicle should also be done so it acts as close as a single piece of metal and by extension the best possible "shield" from the earth below.

Seriously, there really is no secret here, mount the antenna as high as you can, then make sure it is as long as you can get away with.


The DB
 
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The DB said:
Hammy
Click to expand...

Hammie, Hammy? I spelled it wrong, but it was just a figure of speech.

Cap hat loading does make a difference on efficiency and on the lower bands, it can be noticed as I'm talking from experience. The cap hat cancels out some of the inductance losses from the loading coil. The bigger the hat (to a point) the more cancellation.The less coil needed to match a frequency, the less loss and better effective radiation. If the cap hat is designed big enough without being to large, it can nearly match the efficiency of a 1/4 wave whip.

The main point I mentioned this was too show how you can make a short antenna more efficient as opposed to just using a lossy 2 ft or 3 ft. commercially made CB antenna.
 
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