Ok, I know quite a bit about how antennas work. But I am confused by something. I have a magnetic mount for a base loaded whip. The whip was cut for 10 meters. I would like to use it on 11 meters. It's to short but not that much too short. I know that 1/8" difference in length can make a large difference in SWR readings. My plan was to simply add a 3" spring between the antenna and mount, and trim the antenna down a bit more to make it useable for 11 meters. 3" is a lot when it comes to antennas for a specific band. Yet everybody from the CB world says that you can use a spring but you don't have to, and treat it mostly as something top prevent the antenna from breaking if it hits something. Shouldn't adding a 3" spring to a properly tuned antenna, especially one quite a bit shorter than a full 1/4 wave, throw everything off? Wouldn't it have the same effect as adding 3" to the antenna?
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Antenna length
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doctordialtone
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I don't think you'll notice a difference in performance on CB.
mmckenna
I ♥ Ø
Adding a spring will lengthen the antenna. If you look at commercial antennas, like Larsen, the cutting charts have measurements for whip length based on "with spring" or "without spring". If your whip is tuned for the 10 meter band, adding a spring may bring it's resonate frequency down enough to work well on the CB band.
Take what CB'ers tell you with a grain of salt. Not dissing them, that's where I got my start, but there's a lot of misinformation perpetuated in the CB hobby (as well as others…)
Take what CB'ers tell you with a grain of salt. Not dissing them, that's where I got my start, but there's a lot of misinformation perpetuated in the CB hobby (as well as others…)
Like the "Magic" length of coax going to the antenna!
Adding a spring will work. And do not forget about having a ground plane for your mag mount. Preferable in the midle of the car/truck roof.
3" is a lot if you're talking about VHF or above; not so much at HF. Instead of saying "inches of antenna" think "percentage of length" and it will make more sense.
Ubbe
Member
If you have any impedance mismatch that cause SWR in the coax you can minimize the negatives if you get the coax lenght exactly right for the frequency used. A scanner could have its impedance go from 20 ohm to 200 ohm over a frequency band and many antennas does the same and when adding short lenghts of coax, between 1/8 to 1/4 wavelenghts, you could optimize a weak frequency down in the noise to be raised up to a level where it can be fully monitored.
/Ubbe
What about trying one of those CB matchboxes? Not sure if they are still sold.
What about soldering an additional piece of stiff wire to it, then retrimming? You'll need an SWR meter though.
Feel free to shoot down these ideas, hi hi. Just thinking off the cuff
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What about soldering an additional piece of stiff wire to it, then retrimming? You'll need an SWR meter though.
Feel free to shoot down these ideas, hi hi. Just thinking off the cuff
.Thanks for the responses. I just couldn't figure out how adding a 3" spring wouldn't change the electrical length of the antenna like CBrs were telling me. I have always tuned my antennas to get minimal SWR. I have several SWR meters, HF and VHF/UHF. a Bird wattmeter with several slugs, I have a beautiful old Yaesu FT-DX560 that I rebuilt myself, and an Icom IC718. I'm about to leave on a 2000 mile car trip, and I am taking a 2 meter/73cm radio with me, but wanted to have CB as well, in case of an emergency. It's been a while since I have used CB, but I had the radio and all the parts.
prcguy
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At CB frequencies a 3" change in length will move a full size 1/4 wave antenna about 1MHz in range. For a shortened loaded antenna the resonant frequency will change at a different rate if 3" is added below the loading coil vs above it. You might only get a 500KHz change by adding a spring below the coil on a CB antenna but more adding the same length above the coil. Or maybe not depending on the antenna construction.
What about trying one of those CB matchboxes? Not sure if they are still sold.
What about soldering an additional piece of stiff wire to it, then retrimming? You'll need an SWR meter though.
Feel free to shoot down these ideas, hi hi. Just thinking off the cuff
I was rummaging through some cabinets in my garage this past weekend and found one of those. I don't think it's been used for 30 or more years. As you mentioned, I'm not sure if they're sold anymore.
mmckenna
I ♥ Ø
They just hide the issue from the transmitter. Yes, it would work. yes it would allow the transmitter to work better, no it wouldn't fix the SWR issue.
Yep, I should have explained why it's been stored in a cabinet for thirty plus years. Using one will allow you to marry a bad antenna system to a radio but your left with just that.
Have to present another opinion about tuners, mostly from experience. Transmatches, matchboxes, antenna tuners, what have you are used by hams in lots of situations Many times one is presented, especially in the case of multi-band antennas with mismatches that cannot be remedied by simply increasing/decreasing antenna length. If it wasn't for antenna tuners a lot of us would not be on the air. I've used antenna tuners for over 40 years, they are indispensable. They may not be the most prudent way of presenting a 50 Ohm load to the transmitter but a good practical workaround and not something to rule out.
Look at any ham radio catalog, you'll see many kinds of tuners from a number of manufacturers and good number of newer model ham transceivers now have built-in tuners. Evidence of their usefulness in the not-so-perfect world of antennas
Look at any ham radio catalog, you'll see many kinds of tuners from a number of manufacturers and good number of newer model ham transceivers now have built-in tuners. Evidence of their usefulness in the not-so-perfect world of antennas
mmckenna
I ♥ Ø
Yeah, don't get me wrong, they are great for multi band amateur radio use, but for CB where you have a very small slice of spectrum, they only hide SWR mismatches. Proper install and tuning of a CB antenna is possible, and it negates the need for for a tuner. I was just pointing out that an antenna tuner is not a "fix" for CB use, it just hides the problem from the transmitter.
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