First, that 102" is a 1/4 wave at about 27.5 Mhz. 108" is a 1/4 wave at about 26 Mhz. Neither will put you in the middle of the 40 channels without some adjustment. But, what a lot of people don't take into consideration is that the whip isn't the only length you have to work with. The height of the mount, and any spring added also has to be figured into that whip's length.
So how 'tall' is a typical 'ball-mount'? Something like 3", maybe? Add 3" to that 102" and you got 105" of antenna. That should put the thing at about 26.7 Mhz, still a bit low in frequency, so the antenna a bit long. Add that 3" to 108" to get 111", and you are way low, around 25 Mhz, so the thing is much too long.
How do you shorten a 102" whip? The best way of doing that is from the bottom, you don't really want to cut that 'static ball' off of the top. That ferrule at the bottom of a 102" whip is just silver soldered on, so un-solder it. Cut off the length that needs to come off and re-silver-solder the ferrule back on. Fairly simple, right? Yeah, right, till you get to the "cutting off" part, and as already said, a pair of 'dikes' just ain't gonna do it! Several ways of doing that, just pick your own 'poison' and have at it. You might also dig out the torch while you're at it, a soldering-gun ain't gonna do it either.
I understand that MFJ's 108" whip has a ferrule that's held on with set screws. The only thing I'd recommend with those is that after you are sure it's the right length, 'Locktite'em! (Me and set-screws ain't the best of friends, I either get'em too tight or not tight enough. 'Locktite' is my friend.)
So much for that. Now for the 'rest of the story'.
An SWR meter is not the best thingy to use to tune an antenna. It just can't do it without a lot of 'input' (thinking) by the operator. The best you can expect from a well tuned 102" whip, or any 1/4 wave antenna, is about 1.5:1 SWR, unless you use some sort of impedance matching thingy (a device at the feed point of that antenna, or the feed line). That isn't really hard to understand, but sometimes it's a real PITA to -accept-. There are always two parts to tuning an antenna. 1. Making it resonant. And 2. Matching input impedances. An SWR meter can't tell you squat about #1. If you do it right, an SWR meter can tell you something useful about #2. So you have a choice about doing that #1 thingy, throw lots of money at a meter that can tell you about resonance, or go the 'magic' number route. The real world 'magic' number for 1/4 wave antennas is '234', or if you like things in inches, '2808' (234 x 12 inches). That's the 'magic' number I used in the above, so doing the numbers from my example ain't gonna prove nothing. Try actually doing it, and you'll find that it's true though. In fact, it'll still come out a bit 'long', but that's better than too short.
There are NO 1/4 wave antennas that have an input impedance of 50 ohms without some type of impedance matching being done. The same holds true for any antenna, not just 1/4 waves. That's a fact 'Jack', get used to it.
What's that meter that will do all of that stuff?
Wanna guess?? 'Nuther one'a them 'fact jack's. Why doesn't everybody know that? Cuz it wasn't available 20 years ago, at least not at an affordable price.
Do it however you want to do it. But the above works, every time.
- 'Doc
