Can't get SWR on 102 whip below 2.0:1

[maddog355]

I had a set up close to yours in the early 90s on a chrome double/single roll bar. I had 5 of the KC lights on top and made a bracket for the 102" that bolted to the front upright bar just below the turn at the top. 18' of what ever coax radio shack sold to a 29 LTD. The swrs were a little over 2. I swapped to a 12' section of RG8 mini and they dropped to 1.3-1.5 . I have run the same basic setup on 4 fullsize trucks with the 102" on a mirror mount bolted to the front stake pocket between the cab and a alum tool box. The SWRs are never above 1.5. My swrs was actually under 1 before I put the tool box on my current truck. I have always been told 18' for a 102" . I don't understand it, but I know 12' of the mini always worked best for me. Hope this helps.

 

[Turbo T]

Thanx maddog, I actually just now sold the antenna this weekend and sold the light bar a month ago.

 

[Grim Reaper]

make sure the plastic insert on the mount (if like the ones pictured here) is on the antenna side. if not itll be a short, not likly though, swr seems low for that. try putting a 8' piece of rubber hose on the bottom of the whip ,on the chance that the bar and lights are all grounded right. the lights will cause RF reflection. just a thought. might not help. but costs little to try.

 

[Grim Reaper]

A second idea. dbl check your swr s on 40 and one see if one is slightly higher or lower than the other. if higher on 40 whip is to long and adding springs wouldnt have helped. been yrs since i bought a 102 whip. use to be they were a little short, but might have changed over the yrs

 

[KF5FUR]

I am getting ready to mount my 102" whip, I assume it is the same Radio Shack one. I am planning on mounting it to the front panel of the bed right behind the back glass. I have a 03" F-150 with solid back glass. But the antenna measures (mounted to the angle bracket, measuring from the top of the bracket) is 103"-7/16 (8' 7"-7/16) actually its about a 32nd shorter than those measurements. According to N3DNO's wonderful antenna calculator

N3DNO's Antenna calculator

(I plugged in 27.205 MHz or ch 20 @ 1/4 wavelength) it's 8' 7"-7/32 (or a 32nd shorter would be 3/16) and just to be clear I do not have a spring. The base of the antenna near the mount should have no more than 3/4 of an inch of the antenna (or mount nut) within 1 inch of the top of the bed panel and the antenna comes within (while at rest) no more than 3-1/2 inches of the roof. Now with all that said, does anyone think I may have a significant foreseeable SWR problems? Or might I need to add a spring to it? I realize nothing is better than actually setting it all up and putting a quality SWR/Watt meter and antenna analyzer on it but with all that I have read in this thread I am curious.

- Bill

 

[KF5FUR]

BTW I can't imagine what things he may have hit with that thing probably being about 15 ft in height. With his truck being a 4x4, his roll bar is about as high as my roof on my Ford (it was lowered for some ungodly reason by the previous owner) If I mount mine on a mast just above the roof line mine would be no higher than 16ft and would strike every cable/power line and low tree limb on the road outta my neighborhood. And if I am not mistaken is technically a Texas DOT height violation! In Texas I believe all overhead lines are to be 16ft or higher on roads that are not heavy truck restricted, but many lines are lower than that here in my town for some reason. The last thing I want is my whip hitting anything and bending, it's not a big deal to replace a $21 antenna every few months or so but every couple of weeks would be ridiculous! BTW I definitely am putting a tennis ball on it so it wont beat up the brake light near it. You know those tennis balls help lower your SWR ya know.
Actually I heard that years ago from a old CB'er/trucker, and he was being serious!

 

[Turbo T]

KF5FUR, the only things my antenna would hit would be low hanging tree branches, the canopy at the bank, maybe the overhang at Mc Donalds drive thru...I never did hit any power lines or traffic lights with it. I even stopped right under our local traffic light one night at 1 A.M. when traffic was very light, just to see how close I was to hitting it....I had a few more feet to spare.

Also my truck was stock height, not lifted. But ya know, that's the thing with CB, the wavelength and the big sized antenna to make it work...well you "could" go shorter but then you compromise your range...not something I want to do especially running barefoot.

 

[KF5FUR]

KF5FUR, the only things my antenna would hit would be low hanging tree branches, the canopy at the bank, maybe the overhang at Mc Donalds drive thru...I never did hit any power lines or traffic lights with it. I even stopped right under our local traffic light one night at 1 A.M. when traffic was very light, just to see how close I was to hitting it....I had a few more feet to spare.

Also my truck was stock height, not lifted. But ya know, that's the thing with CB, the wavelength and the big sized antenna to make it work...well you "could" go shorter but then you compromise your range...not something I want to do especially running barefoot.

Your last sentence goes for me as well. That's good to know that you didn't strike any lines goin down the road. I did read a few minutes ago from this post that those R. Shack whips are tough as hell, and the experiences some guys had and them hitting stuff. I read somewhere here that a guy hit a overpass with his at 60 Mph and it did no noticeable damage, that's reassuring.

I found this interesting http://www.worldwidedx.com/cb-antennas/87429-102-whip.html

post number 29 about a DC grounded matching coil, it is supposed to work wonders for 102 whips, no spring, no GR riser, with a MFJ antenna analyzer checking it over you could possibly see flat SWR and perfect matching, that is probably ideal mounting conditions though.

Just wish I had the details to make one for 11 meters, anyone know?

 

[KF5FUR]

KF5FUR, the only things my antenna would hit would be low hanging tree branches, the canopy at the bank, maybe the overhang at Mc Donalds drive thru...I never did hit any power lines or traffic lights with it. I even stopped right under our local traffic light one night at 1 A.M. when traffic was very light, just to see how close I was to hitting it....I had a few more feet to spare.

Also my truck was stock height, not lifted. But ya know, that's the thing with CB, the wavelength and the big sized antenna to make it work...well you "could" go shorter but then you compromise your range...not something I want to do especially running barefoot.

I was mistaken on the height I stated earlier. I had the wife help me measure it. Mounted to the bed makes it about 12' 6" off the ground, on a mast putting all the antenna above the roof puts it at 14' 6". Either way, no drive thru's for me as most are 9'. Also when I get the stock lift back on the truck that will put it at around 13' 2" and 15' 2" with the mast. Decisions,decisions...

 

[maddog355]

KF5FUR You should not hit power lines, or have swr issues. As I posted mine is mounted at the front drivers side stake pocket between the cab and tool box. My truck is lifted (44" tires) and I hit alot of limbs and some gas station awnings but I don't even hit my supply line between the house and pole. My swrs are about 1.5 . I always run a spring, The isolator washer will not last without it. Hope this helps.

P.S. I can even catch the tip of the whip in the rear steak pocket for drive up windows.

 

[KF5FUR]

KF5FUR You should not hit power lines, or have swr issues. As I posted mine is mounted at the front drivers side stake pocket between the cab and tool box. My truck is lifted (44" tires) and I hit alot of limbs and some gas station awnings but I don't even hit my supply line between the house and pole. My swrs are about 1.5 . I always run a spring, The isolator washer will not last without it. Hope this helps.

P.S. I can even catch the tip of the whip in the rear steak pocket for drive up windows.

I went ahead and bought the pot belly spring which brings the length to about 107"-11/16 as measured from the top of the angle mount. Now just gotta work out the Inductive Matching DC ground coil running between the antenna stud nut and grounded close to the connecting location to the shield ground strap that goes to the body of the bed. Like this... (borrowed from K0BG's site)



"The coil at left has 9 turns, is 1 inch inside diameter, and wound with #14 Thermalese® (enameled) wire. The coil's form factor should be kept close to 1:1 (length to diameter). Long skinny coils do not work nearly as well. You can also use building wire, but it is a little harder to work with. In actual use, the turns are spaced a little further apart to adjust the inductance. The coil needs to be about 1 uH, but in the real world, the value may be between .5 uH and 1.5 uH depending on the actual input impedance at resonance." (this is also from K0BG's site) He is talking about mainly the remote control screwdriver antennas but those cover 10 meters on down, so this should be adequate for 11 meters as well, the coils inductance can be slightly manipulated by stretching the coil if needed. Here is a good pic on what I assume is a roof or trunk mount.



While I cannot connect the shield like his on my typical angle bracket mount, I will drill and tap to the hex of the so-239 stud and connect the grounding strap via a crimp end and solder the wires to connector for good measure as well. The coil will connect the same way to the stud nut then the other end will connect to a SLU-70 lug/wire connector to the bed of the truck within an inch of the ground straps mounting with the same lug.
Then I will proceed to check the beds grounding, I remember seeing a while back while under it several straps to the frame for the bed and cab, it being a 03' F-150, it should be grounded like this anyway. I will also check with my multimeter the amount of resistance between the bed/frame, bed/cab, and cab/frame, also add a strap to connect the bed and cab if necessary.
I think I have covered all basis there, lemme know what yall think.
Added note: Many folks think this is a RF short, it is not, it is a electrical short. The inductance of the coil prevents it from becoming part of the antenna. I have seen small coils soldered from the back of a bulkhead connector to the body of a unit, don't remember what the unit was for, but I assume it was the same principle.

-Bill

 

[maddog355]

I am a simple country boy, So I don't understand the purpose of the extra coil mounted to the body? I will be the first to admit I am not very technical so please explain.

 

[KF5FUR]

I am a simple country boy, So I don't understand the purpose of the extra coil mounted to the body? I will be the first to admit I am not very technical so please explain.

Check this guys site out, he's a Ham genius, a lot of what he is saying applies to CB as well. Antenna Matching

Focus on the "Matching Input Impedance" and "Inductive Matching"
Any questions about that read there I will try to answer, if I cannot, start a new thread, I always get fast replies on this site, I said it before and I'll say it again, I love this website!

 

[maddog355]

After reading that page there is something that stands out in my mind.

" if your HF antenna doesn't require matching to provide a low SWR (with the possible exception of 10 and 12 meters), then you need a better antenna, a better mounting position, or both! Further, proper matching reduces the chances of RF flowing on the outside of the coax feed line (common mode currents). "

As he is a ham and the page is ment as information for other hams, The "exception of 10 and 12 meters" would likely include 11 meters as well.

Basicly I am wondering if you arn't over thinking a problem that don't exist?? I would love to hear some other members thoughts on this.

 

[W5LZ]

MD355,
I guess it depends on which part you think is being "over thought". If it's the inductive matching that you mean, then no, it isn't being 'over thought'. That's a very common way of matching impedances used by a lot of manufacturers, not just for ham antennas. If it's something else that's being 'over thought', I don't understand what.
- 'Doc