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High SWR with a 102" whip

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SamB
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High SWR with a 102" whip

Post by SamB » Saturday, 13 May 2017, 20:12 PM

I just installed a 102" SS whip with a spring to the front of my CJ7, it's on a homemade mount, made out of mild steel, and the coax is from rat shack. I ground the paint off the core support where the mount bolts to it and I also ground the protective coating off the mild steel mount where it contacts the core support and where the antenna connector grounds to the under side of the mount. The radio I'm using is a cobra 19 that doesn't have an SWR meter on it. The SWR meter that I'm using to get my readings from is a Midland 23-500 from the late 70's or early 80's. When I check the SWR with the meter I have I'm getting a reading of around 2.5. I'm trying to to figure out what part of my setup is causing the high SWR readings, or if my meter is bad and my SWR is good? Any suggestions on what to check or change would be appreciated.

On a side note, does my mounting location look like a good choice?

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sonoma
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Re: High SWR with a 102" whip

Post by sonoma » Saturday, 13 May 2017, 21:52 PM

from the pictures you have posted the antenna is either touching the body or almost touching. it can not do that or you have a very high swr reading. going down the road it will be right against the body. this will not work.

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Re: High SWR with a 102" whip

Post by 443 Arizona » Sunday, 14 May 2017, 5:36 AM

:) hello SamB, what kind of radio reports do you get? any transmitted noises ? and when in receive mode, do you get engine(electrical) noises?
these noises can be from mounting it so near the engine compartment.

i would mount up top.

or, maybe the ratshack coax is not shielded enough, or runs too close to other electrical, or squeezed,pinched,nicked,(etc.) allthou i give you credit for not doing that kinda stuff, it has to be considered.
one trick to try is , using plain ol' string, tie back the ant. to a position simulating the typical deflection that will occur and see what your SWR is.
another overlooked problem that can occur with the big spring is corrosion and lack of "electrical connection", should be a piece of wire going from the top to the bottom of the spring,( on the inside).
another little tidbit,,, is the ball on the tip? and , sure it's 102"? 108 or 109 with spring? :think: (i think thats right) is the (ant.)mount insulated enough? can you try a different radio to compare?
are you close to anything while checking swr?
radio chassis grounded?
p.s. is that mounted to fiberglass? i just enlarged the pix and cant tell.
alot of monkeying around but it all adds up.

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SamB
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Re: High SWR with a 102" whip

Post by SamB » Sunday, 14 May 2017, 12:59 PM

The very bottom of the spring on the base of the antenna is only about an inch from the hood when it's closed, I hadn't thought about that. Here's a pic.

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As far as the coax, I think it is in good condition, but it may have issues, I don't have any coax to swap it out with and check at the moment. I also have about 5 feet of extra coax looped up in the engine compartment, I don't have a good route for the coax to stay away from everything electrical if I mount the antenna to the front of the Jeep. The antenna mount is mounted to steel, and the tip is still on the antenna. I'll get my muti-meter out and check for continuity of the spring, I have had that be an issue before. I do get a lot of engine noise on the radio when it's running, I've tried to ground the chassis, but maybe I haven't done good enough. I may end up moving the antenna to the back of the Jeep if I can't get the noise issue fixed.

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Re: High SWR with a 102" whip

Post by sonoma » Sunday, 14 May 2017, 13:35 PM

if the coax is running with the main wiring harness for the jeep you could be picking up noise there also. a loop in the engine area is not good either. while driving down the road the spring of the antenna has got to be getting closer to the hood also since it will lean but from the wind. back a few years ago my son had a 82 jeep CJ7 and what we did for a antenna on his jeep was we made a bracket that bolted to the frame under the right rear corner and made it where it stuck out about 6 inches from his spare tire. we used the whip as you have. we routed the coax up to the front seat area where he mounted the radio and the swr was about 1.5 and it worked just fine. we used some 2 inch angle iron to make the mount and just drilled a hole in it for the antenna stud. he ran it like that for about 5 years until he bought a new pickup and it was a good setup. just some thing to think about.

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SamB
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Re: High SWR with a 102" whip

Post by SamB » Sunday, 14 May 2017, 15:42 PM

I just found some old coax I forgot I had and used it to hook the antenna up. I ran it from the CB out through the door to the antenna, completely avoiding the engine bay and any wiring, I also removed the spring. My SWR dropped from 2.5 with my old setup to 1.5, I then started the engine and got the same amount of engine noise through the radio. I've decided that the front of my Jeep is just too noisy for an antenna. I'm going to move the antenna back to the right rear corner of the Jeep, and possibly go with a top loaded fiberglass antenna, the old whip can really get to whipping off road. Once I settle on a mounting location I'm going to shorten the coax so I don't have to loop excess anywhere, and replace it if I still have problems. I'll report back once I get everything moved. Thanks for all the help.

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Re: High SWR with a 102" whip

Post by carrier » Sunday, 14 May 2017, 22:05 PM

Resonance of an antenna is one thing, Impedance is another. Most likely your antenna is close to resonance but maybe is not giving you a good impedance match. Try installing trial and error silver mica capacitors one at a time across the coax center wire at the antenna feed point with the other end of the capacitor to ground. (Across the coax at the antenna feed point base.) Try a 5 pico farad, then a 10 pf and then a 15 pf and then a 20 pf capacitor one at a time. If the SWR keeps going down, keep increasing the capacitor size until they no longer reduce your SWR. (Small 500 volt ones will work here just fine. You can get them from just about any electronic parts house.) Having the base spring close to your auto body does not help things.
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Re: High SWR with a 102" whip

Post by MDYoungblood » Tuesday, 16 May 2017, 5:36 AM

That is a cool trick that carrier posted, a lot of base antennas have a cap or shunt to balance the impedance. 99% of the problems found using Radio Shack coax is they do a lousy job with installing the connectors, (one of the reasons I like installing my own connectors), the cable itself really isn't that bad. As far as the noise from the engine, it depends on whither it is static (spark plugs), whine (alternator), or a steady noise (fuel pump), all can be reduced with patience and help from our knowledgeable members.

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