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Whacky SWR's on Mag Mount Antenna

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forkeye

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Ft. Lauderdale, FL
Okay so this has driven me crazy. Its the only car I have the problem with so here it goes.

I bought 3 K30 Magmount antennas from K40 as part of a promo run by the Pilot travel centers.

I use one on rental cars when I travel. I use the other in my personal vehicle. The third is a spare.

Antenna 1 (the rental car antenna) works like a champ. SWR's are always a steady 1.5:1 no matter what vehicle I am renting. If there's metal on the vehicle, it sets down, run the coax into the passenger compartment. Presto! Great antenna. Was in the Mojave and talking to Kenosha, Wisconson a couple of weeks ago. Great antenna running barefoot on a chintzy Midland 75-822 handhelld unit with the car adapter running in Hi Power (4 watt PEP) mode. Great little portable radio, great for travel; great weather radio. I do a lot of highway driving so this radio with the Anytone PTT mike package is great. Just like having a real CB attached to the dashboard.

Antenna 2 (personal car antenna) has crazy SWR's. Doesn't matter where I put it on the car. Roof, trunk lid, etc. It starts out fine, a couple of bumps in the road, then its a nutty SWR. At one point it was banging 7:1. I am picking up Skip, radio has not blown finals, nothing.

As soon as I open the car door or the trunk lid, SWR's are fine. No explanation. Run the coax through the window, and roll it up a bit, SWR's are crazy again.

So ... enter Antenna 3.

I replace Antenna 2 with my spare (Antenna 3). I'm getting the same whacky SWR's.

I write to K40 Corp. Tell them the above. They write back saying the radio/antenna are not grounded. So I start searching for some kind of solution to this problem. Suggestion is a grounding block to the car's chassis or body. I check my trunk lid, its grounded throughout. Nice old Crown Victoria with metal all over the place. I ground the radio to the inside ground. Same problem. Crazy SWR's. So there's no ground issue there on the inside. Suggestion is turn to a grounding block. Okay, this makes no sense. The coax is 18 feet for a reason. Like the firestick and other no-groundplane antennas, you do 2 quarter waves with a quarter wave on top. The shielded coax is supposed to act as counterpoise to avoid ground. So why use a grounding block? Alas! They don't make a grounding block. So I get a bulkhead mount double SO259 and fashion a ground block using #0 conduit standoff, and voi la, now I have a grounding block for the car body. Run my coax through to the trunk, splice in two PL 259's as suggested.

What happens? I have a super-RF-grounded antenna --- BUT!!!! ... Big But....--- I still have the same problem! Open a car door or pop the trunk, SWR's are fine. Close the door or trunk, boom! SWR's are high.

This whole drifting SWR thing got me rattled. So ... I make an RF air choke to make sure I don't back-end my finals. Nada. No difference. I've wasted more coax.

It works fine with the trunk popped slightly open, but ya know .... not conducive to highway driving and city driving isn't much better. Besides I have stuff in my trunk I don't want taken out of there at a stop light or stop sign.

I have also checked my coax. (A) Hardly any loss. Conductivity is about 99.9999% using my VOM. (B) No shorts. None. Even with the trunk shut no shorts. Zero conductivity between shield and center conductor. Popped open no difference. (C) Checked my patch coax. Its fine. No shorts.

Scratching my head, my nose, my nethers, switching hands and still can't figure this one out.

Been around CB and Amateur Radio since the early 70's, heck I still have my old CB "KWR" license even though they stopped renewing them, oh about 1978 or so.

Short of buying a $250 antenna "tuner" device or an autotuner, any suggestions here?

BTW I just bought a cheapo antenna matcher on Ebay hoping that might just fix the problem and get it over with. I had toyed with a 250pf - 1250 pf variable cap and making a resonance tuner for it, but before I spend the mucho bucks tracking one of those down, maybe someone's got ideas here?

PS -- K40 has no solutions to this issue. They can't explain this. And PPS -- (A) I thought about the Faraday Cage effect but ruled that out because I didn't see RF bouncing around using the FS function on my SWR/Watt/RF Meter. And using RF detector could not find any "leaks". (B) I toyed with the possibility that there is an electrical "short" when the trunk light switch engages or the door light switch engages. But when I manually depress them and check the SWR's, no difference. So its not the electrical system playing tricks on the RF ground.

Thanks much.
 

FiveFilter

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Jan 1, 2016
Messages
228
Any short antenna can be a little more tricky to tune because they are not very broad-banded, but I've had great results with my K30 both on the trunk and on the roof of a little Mazda3 sedan. I only use a CB for long road trips and only on Channel 19, so I quit tuning / cutting my K30 when it reached below 1.5 SWR on Channel 19 on the Mazda because I didn't want to cut it so much that it wouldn't tune as well on other vehicles I might use it on.

The relatively large counterpose presented by a big ole Crown Vic should be helpful when tuning the K30. The problem is not the antenna per se, because the same effects are present in two different ones and, I assume, the rental-use-dedicated K30 too. So it has to be something relating to that particular Crown Vic.

Whenever I've heard of performance improving when opening a trunk or a door, it seems like the usual suspect is a grounding issue often causing an excessive common mode current effect. But you've assured that the car is well-grounded and no measures in that regard has helped. That is a mystery.
 
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forkeye

Member
Joined
May 28, 2017
Messages
9
Location
Ft. Lauderdale, FL
UPDATE: Well the one antenna issue was solved in the Crown Vic. I chopped the coax, disconnected the ground braid entirely and kept 2 feet of coax off the antenna. I put a #0 conduit stand inside the trunk, screwed it into the trunk lid frame, ran a 15" grounding strap from the conduit stand to the vehicle body, and made a positive ground out of the entire car body. I also put a soldered PL-259 with adapter on it. So the only possible thing that ran through the entire coax was the radiating 50 ohm center dialectic.

So to be clear, I have the wire whip, attached to the coil, attached to 2 feet of coax, which now runs inside the trunk to the #0 conduit stand. I then inserted a 4 inch bulwark mounted double SO-239 (F/F) which tightened down well on the conduit stand. I then ran a fresh 20 foot run of coax from the front to back of the vehicle so NOW I actually have enough straight coax running to the radio which I can put on hump or under the ashtray. Connected the coax to the other end of the bulwark double SO-239.

Now there is nothing loose, no way its not grounded. Its grounded to just about everything except the dog riding in the back seat. I checked the tuning both with and without the 20 foot coax run. Its all the same SWR in a stationary position.

Jiggled it, and torture tested it with a bunch of trunk slams, etc. Drove over pot holes. Its all so far so good.

I'm getting great reception. Watt meter is showing a solid dead-key at 5 watts (its an older radio).

SWRs are showing a consistent 2:1 give or take a teensy budge.

Let it sit overnight, next day SWR's are 1.5:1. Even better. Can't figure out for my life why its better one day and not the day before. Wait a day, now its back to about 2:1. Now same spot, same parking space, and nothing within 50 feet of it.

Then the other day its flip-flopped again and back to 2:1. But this time I noticed the check engine light go on just before I checked it.

So I ran a diagnostic scan on the car. And what I think is happening is that a sensor is showing odd voltages, and that may be the reason why. Its not a complete short, so maybe the sensor is shorting a little or there's a short and its feeding some RF or electricity to the ground that maybe ought not to be there? Don't know. So that's a question for the mechanic and if and when I swap out that sensor, maybe it'll be consistent. But I'm thinking that's the issue. There's some short in the electrical system and its causing enough problems to mess up the SWR's.

Electrons have a mind of their own, and so do electronics. So its a possible.

Meanwhile I tested my other K-30 on the same car and got wildly swinging SWR's. Then I jiggled the coax right where it goes into the antenna base and discovered the issue. Cheap coax. There was a break in the center dialectic so that is why it was swinging wildly. The two antennas had similar issues, but different causes.

In either event the K-30 is about as grounded as possible. With the other one, I took the magnet out, got the coil out, replaced it with some regular RG58/U not the polyfoam shielded stuff that K40 uses.

Its consistent but when bench testing it with my other radio, the radio is showing a crazy high SWR, but when using it with my handheld, which tested fine against the other antenna and consistent, it showed 2.5:1 SWR's just not on the car. Sitting on a tool chest.

Once again its a tossup. I am thinking that there is another issue. There's a capacitor in there that runs across the ground and coil I guess to tune to resonance. I am thinking that's another hob goblin in the works. Ordinarily the cap should be in there, but if its bad it could also be an issue.

I checked with K40 and the same coil is in the Lil' Wil antenna, and they are basically identical antennas so it seems. Same type of coil, same cap, same arrangement. Not sure why one is more expensive or different than the other.

So its all a mystery. I'm thinking of going with the Stryker double-barreled mag mount. Pricey but they guaranty really good results.
 

forkeye

Member
Joined
May 28, 2017
Messages
9
Location
Ft. Lauderdale, FL
Second Update

Finally got an answer. 180 pf capacitor in there was bad, but why it went bad was another issue. So its replaced.

The issue was a confluence which proves once again you can't assume only 1 thing causes a disaster.

Paint is very thick so inductance between the car body and the antenna base was bad. That was fixed when I added a direct ground connection and strap to the chassis like I described. What blew the capacitor was a bad sensor that electrically created some issues. How exactly is not known, but it was creating all sorts of issues for all of the electronics associated with the car's system.

Third confluent issue was a frayed wire on the cigarette plug where I was getting power from.

Now the SWR's bottom out at Ch. 20 nicely. Lots of work to figure out why. So to get this straight, a sensor in the car went bad, caused electrical problems, causing the cap to go bad in the antenna. Also the paint is too thick so there was poor inductance between the antenna base and the chassis. And finally a frayed wire on the cigarette plug where the wiring was coming in, added more troubles.

Yep. 2 or more issues can cause a disaster.
 
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