Different Look At 102" Whip Discussion

[Marconi]

most of us CB'er only have a vswr meter for checking/tuning antennas . folks have been getting by with just that for decades . using a analyzer could do a better job though . on my mobile my 10k tunes to 1.3 on a mfj259b at the shop , but take it off and put on a 102 whip and it shoots above 2.5 .
resonance smesonance ....... i refuse to try to run a radio into that kind of vswr , so i stuck with the 10k .

BM, do you really think a 102" whip will show a 2.5 SWR if the ground plane part of the installation is suitable? I've heard similar stories for ever, and that is why I decided to do my 1/4 wave work in the summers of 2003/2006... trying to understand what goes on with a 1/4 wave radiator.

IMO, the 10K raised coil antenna is less effected by the physical ground plane presented, or said another way, it just doesn't need as much ground plane to allow it to work effectively...as long as the coil is effective and well in the clear.

Your own experience with your homemade Starduster type ground plane should give you a clue in this regard, because I think you saw a good match. What else besides the ground plane is possible to cause such bad results in your mobile install, assuming it's installed correctly?

A 1/4 wave radiator requires a good ground plane, and what you get or think you get with a mobile in this regard...is almost always a little less than suitable.

The open coil mobile type antennas are just more forgiving with mobile installs with a poor ground plane.

Compare the match and SWR with these Eznec models below and you'll see what is likely to happen as the ground plane becomes less effective with a 1/4 wave radiator. Your mobile SWR problem probably has little to nothing to do with the radiator itself.

These three models shown are with the same radiator and the radials are:
Horizontal
Slightly slanted like a mobile might present
Slanted less than 50* degrees like your SD'r might have presented.

View attachment Bm's 102'' radiator idea..pdf

 

[gamegetter]

fwiw

my 102's have always matched well, always 1.5 to 1 or less.

 

[maddog355]

When I first started using a 102" I had in on a ball mount with about 20' of coax and no spring. I don't recall what the swr was but they were acceptable for a stock radio. But I started having trouble with the mount grounding out. When I set up my next truck I made a mount that mounted to my rollbar and used the plastic washer from a standard mirror mount but still no spring. I kept breaking the washer and finally someone told me it needed a spring. Sence then I don't recall breaking any more washers. So I think the whip is actually made short for use with a spring to reduce stress on the mount.

 

[BammBamm]

I made my own extension with a 24" long 3/4" piece of copper tubing and end caps and 3/8 x 24 bolts I used bolt cutters to cut my 102 down to tune a perfect match. No spring in a ball mount on the rear corner of my car. I wanted it on the roof but the sliding sun roof stopped that idea and my trunk deck is fiberglass.

 

[HomerBB]

I would say something useful in this thread, but others are already doing a decent job. I will interject, however, that a mobile install uses the automobile as a counterpoise, and the earth beneath the car as the ground plane. I have had the same antenna change impedance from good to bad, or bad to good or better simply taking it from a fender mount, or trunk mount to the roof, or any combination of the moves. I have even seen an antenna that had a great impedance in one place be virtually unable to be tuned buy going to the roof from the fender, and not in the case of a SS whip.

The antenna below matched very well on the fender,and although there was a fair amount of discussion surrounding it when I made it, it will not exhibit a good SWR on the roof puck. So I am waiting for my MFJ-259B to arrive to figure it out. It shipped today.

Maybe I haven't gotten too far from the thread subject. . .

 

[nobody]

When I first started using a 102" I had in on a ball mount with about 20' of coax and no spring. I don't recall what the swr was but they were acceptable for a stock radio. But I started having trouble with the mount grounding out. When I set up my next truck I made a mount that mounted to my rollbar and used the plastic washer from a standard mirror mount but still no spring. I kept breaking the washer and finally someone told me it needed a spring. Sence then I don't recall breaking any more washers. So I think the whip is actually made short for use with a spring to reduce stress on the mount.

Early in high school I tried using a 102" on a ball mount with no spring. I noticed the antenna kept getting loose a little. Then one time I hit a limb and it stripped the threads out in it. But with the spring I never had any problems with it.
As for a SWR reading, with every 102" whip and spring combo I had excellent readings on the meter. The very best readings I ever had was with the same combo mounted to the back side of and in the center of a tool box mounted on a Toyota 4X4 truck (the closest I had it to actually "flat lining").
Currently I have it mounted to the driver's side right behind the cab of my Dodge Ram. The SWR reads something like 1.2 to 1.3. I'm going to buy a tool box fro the truck just so I can get them lower to 1.1. But yeah I never had to worry about my 102" and spring and my radios. Never had to trim them or anything. Never to short or to long for any CB channel. The few others I tried I never could get them tuned across the band like that. I even ruined one with a mag mount set up.
BTW I thought the 102" was designed to be used with a spring for it to be tuned correctly? The time I ran one with out it I didn't have a meter and knew very little to nothing about this stuff. Yeah it a little annoying at the drive throughs but I can live with that.

 

[Happy_Hamer]

Maybe I am wrong, but I thought in the past I bought a 102 inch antenna from the rat and just for the hell of it I measured it and it was only 96 inches, I went back to the rat and he said technically it is supposed to be mounted with the 6 inch coil to equal 102?

I have the MFJ 108 and I have not measured it but now I will be in the garage in the morning and take some measurements, curiosity,,,,,,,,,,,,

I have one "102 inch" still in the cardboard tube from the rat so I will measure it,,, this way I know it has never been cut.

Anyone keep one near them that they can measure tonight LOL?

 

[maddog355]

Currently I have it mounted to the driver's side right behind the cab of my Dodge Ram. The SWR reads something like 1.2 to 1.3. I'm going to buy a tool box fro the truck just so I can get them lower to 1.1.
BTW I thought the 102" was designed to be used with a spring for it to be tuned correctly? The time I ran one with out it I didn't have a meter and knew very little to nothing about this stuff. Yeah it a little annoying at the drive throughs but I can live with that.

When I had mine mounted on the rollbar the needle never moved when I would check the SWR, Someone told me that SWR being too low was a bad thing so I went to 12' of coax and that got them to 1.3. As for drive throughs, My truck is not drive through friendly anyway.

 

[W5LZ]

A common misconception is that the 102" whip is made for CB/11 meter use. It isn't, it was around a long time before the CB/11 meter band was. It's also used in several other services besides CB, and with modification can be used almost anywhere. It is intentionally made longer than needed for most bands, 11 meters included. Why? Cuz it's easier to shorten than to stretch the thing! Most antennas you find are made that way, it's just common sense.
Something to consider is that the "antenna" begins where the feed line ends. That means that anything attached to the end of the feed line, mounts, springs, etc, are included when finding the length of the antenna. A typical ball-mount is 4" long, so is a typical spring. Start adding up all those things and see what the length of you antenna is. Now do some figuring and find what the resonant length is for somewhere in the 11 meter band. I think you'll find that most antennas using those devices are too long to be resonant where you think the thing is resonant.
That resonant length depends on how/where the thing is mounted. There are a lot of things that affect resonance, it's not strictly a matter of length. The typical surroundings of an antenna do affect it, resonance being only one of the ways. It really isn't as simple as most people think.
And then there's the -fact- that a resonant antenna very seldom (if ever) has an input impedance of 50 ohms. so, it's a simple matter of changing the input impedance and there are a number of ways of doing that. Unfortunately, it's not a "plug-n-play" thingy, you have to know what you are doing and how to do it.
And then there's the -fact- that an SWR meter can't be used to find or determine resonance. It has no idea what resonance is cuz it can't measure or compute reactance values. Why's that important? Because reactance is what determines resonance, and reactance doesn't radiate anything. It's not something that you want in an antenna. It can come in very handy in other places, but NOT in an antenna.
Think about that for awhile...
- 'Doc

 

[gamegetter]

for 1/4 wave

234 / f (in Mhz) = feet

234/27.205=8.6 feet

102/12"=8.5 feet

 

[Wire Weasel]

Maybe I am wrong, but I thought in the past I bought a 102 inch antenna from the rat and just for the hell of it I measured it and it was only 96 inches, I went back to the rat and he said technically it is supposed to be mounted with the 6 inch coil to equal 102?

RS used to sell more ham gear including 10 meter mobiles and associated supplies. I also remember their offering a 96" whip and that clearly was for 10 meters.

I have still never seen a 6" coil spring anywhere excepted for some big tough military piece.

W5LZ and some of the others are correct also with the many comments regarding satisfactory grounding and placement issues. Every installation is going to be different and one will have to just "see what works" on any particular vehicle and antenna arrangement.

Power output is also going to be a factor , especially with adequate grounding issues. A setup that appears to work well with 4~100 watts power may see the SWR go off the chart if you try to slap some big 8 pill monster in there.

You just have try things and see what works out on each and every installation and do what is electrically necessary to make everything happy.

 

[W5LZ]

gamegetter,
Right, and that 0.1 foot is 1" and a little bit. So, 102" + 1" = 103", not 108" or whatever. The only 1" mount I've ever seen was one of the 'acorn'/'beehive' things. Most of those I would have doubts about holding a 102" whip, so if using a bigger/taller mount, then I'd cut that 102" whip down a bit to get it to the 'right' length. Does that mean it'd have a real nice SWR? Nope, but I'd bet it would be within reason.
- 'Doc

 

[gamegetter]

folks get hung up with swr and try to match by cutting the antenna and throwing resonance off.

 

[psycho]

When I had mine mounted on the rollbar the needle never moved when I would check the SWR, Someone told me that SWR being too low was a bad thing so I went to 12' of coax and that got them to 1.3. As for drive throughs, My truck is not drive through friendly anyway.[/QUOte

How can SWR be too low? CB stands for Continuous Bullshitting.

 

[W5LZ]

The idea, not having the SWR get too low, was a justification for not wanting too good of a signal so that you would be under suspicion of doing something wrong. Sort of like, where did you get all that money you're spending unless you robber a bank, you know? It really doesn't work that way.
- 'Doc

(unless you happen to be a bank robber)