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CB: Installed new antenna, now SWRs high on every channel

C

COLTON_CJF

New Member
May 20, 2023
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Before judging for my actions, I just want to make it clear I am pretty oblivious to every aspect behind the science of radios. So here's my issue.
I have a Midland mobile (magnetic) CB antenna that I use permanently on my truck because it works great with my Midland radio. Reason I have a CB in the first place? It's just a cool addition. Now, it is a great antenna except the fact that the thin antenna piece keeps breaking for no reason right where it inserts into the big coil piece (3rd time it's happened). It is probably 1/16 or 3/32 diameter and 13" long. I'm done replacing it so I decided to replace it with my own stronger 1/8" mild steel wire. I used a tap and die to fine thread it in the center loaded coil part, so shouldn't have any connection issues there. I spray painted it aluminum to match the rest of the antenna and it looks great. It receives awesome, does not transmit! Once I went to tune it the SWRs are so far gone I can't even get it to the calibrate mark turning the knob all the way to the left. All I know is the longer the antenna the better I think every extra foot you can expect 1-2 miles more of transmit power. Why don't I just try to cut it down? Because I don't believe that's the answer for how out of tune it is.
Here are some measurements I took of my antenna that might be useful:

Stock Antenna 13"
Total height 28"
Top of base to coil is 12"
The coil is a bunch of small copper windings about 2" tall, just in case that some how affects the height new antenna can be

Home Made Antenna 35"
Total height 50"


My antenna I have that had great SWRs
Amazon product ASIN B0000ATNH3
 

Hmm. From the sound of it you have moved the resonant frequency of your antenna lower. Probably a lot lower. The physical dimensions and the exact number of turns in the wire coil together are designed to resonate at 27 MHz. If I understand your description, you have made parts of it a lot larger than before. Longer length elements will resonate at a lower frequency. A quick test with an analyzer that goes down to ten MHz or so might show this, but your radio does not cover frequencies low enough to see the SWR fall below the reading you get now.

Size matters when it comes to the elements of an antenna,

73
 
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nomadradio said:
Hmm. From the sound of it you have moved the resonant frequency of your antenna lower. Probably a lot lower. The physical dimensions and the exact number of turns in the wire coil together are designed to resonate at 27 MHz. If I understand your description, you have made parts of it a lot larger than before. Longer length elements will resonate at a lower frequency. A quick test with an analyzer that goes down to ten MHz or so might show this, but your radio does not cover frequencies low enough to see the SWR fall below the reading you get now.

Size matters when it comes to the elements of an antenna,

73
Click to expand...
Makes sense since I'm sure the copper coil was made for the exact antenna length it came with. I'll try trimming it down a bit at a time and maybe find a good length that will work with the radio. If not, ill trim it down to the 13" the last thinner antenna was. From my understanding, it's the length that matters, not the "thickness" of the antenna, right? Because my 1/8" rod is a lot bigger and sturdier than that 3/32 crap was. Guess i'll find out tomorrow or whenever I got time to try and figure if I'm going to have a working cb or if I totally screwed it all up, haha.
 
This is where an antenna analyzer pays for itself. Well, if you have to do this more than just once, anyway. If the antenna has been 'retuned' to 19 MHz or 23 MHz for example, an analyzer can show you the difference it made from shortening an element. Lets you zero in on a target frequency.

A radio alone is only useful for this if the antenna's resonant frequency is close to the limited range of channel frequencies that the radio covers. I would try a length as close as you can to the original. Making the element fatter will also affect the length adjustment, but this would at least get you close.

73
 
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Reactions: COLTON_CJF and Crawdad
nomadradio said:
This is where an antenna analyzer pays for itself. Well, if you have to do this more than just once, anyway. If the antenna has been 'retuned' to 19 MHz or 23 MHz for example, an analyzer can show you the difference it made from shortening an element. Lets you zero in on a target frequency.

A radio alone is only useful for this if the antenna's resonant frequency is close to the limited range of channel frequencies that the radio covers. I would try a length as close as you can to the original. Making the element fatter will also affect the length adjustment, but this would at least get you close.

73
Click to expand...
I'll have to look into that especially if I end up getting another rig that I'd like to mount dual whips instead of a short magnetic antenna. Anyway, I did a lot of trimming (about 2ft worth) that was slowly bringing me back down from the red area (3+ swr). Eventually it ended up balancing out at about 1.2 SWRs for ch 1 and 40 and a sweet spot of 0.5swr in the middle channels like 10-30. It ended up shorter than my last rod at about 11 1/2" instead of 13" which is probably the thicker rod being proportionate to the longer length of the last rod compensating for the small diameter of the last one like what you were talking about how sizes will affect it a wee bit. I learned a lot since I've always been told the more length on an antenna the better considering that's not always the case! I appreciate you all very much for your time devotion helping me figure this out! Good night!
-Colton
 
COLTON_CJF said:
I'll have to look into that especially if I end up getting another rig that I'd like to mount dual whips instead of a short magnetic antenna. Anyway, I did a lot of trimming (about 2ft worth) that was slowly bringing me back down from the red area (3+ swr). Eventually it ended up balancing out at about 1.2 SWRs for ch 1 and 40 and a sweet spot of 0.5swr in the middle channels like 10-30. It ended up shorter than my last rod at about 11 1/2" instead of 13" which is probably the thicker rod being proportionate to the longer length of the last rod compensating for the small diameter of the last one like what you were talking about how sizes will affect it a wee bit. I learned a lot since I've always been told the more length on an antenna the better considering that's not always the case! I appreciate you all very much for your time devotion helping me figure this out! Good night!
-Colton
Click to expand...
Dual whips? On what vehicle?
 
nomadradio said:
This is where an antenna analyzer pays for itself. Well, if you have to do this more than just once, anyway. If the antenna has been 'retuned' to 19 MHz or 23 MHz for example, an analyzer can show you the difference it made from shortening an element. Lets you zero in on a target frequency.

A radio alone is only useful for this if the antenna's resonant frequency is close to the limited range of channel frequencies that the radio covers. I would try a length as close as you can to the original. Making the element fatter will also affect the length adjustment, but this would at least get you close.

73
Click to expand...
Not everyone can afford an analyser.
 

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