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Tuning a 102"

AudioShockwav said:
A 1/4 wave whip the correct length will typically exhibit slightly low impedance in a mobile set-up, but almost every install is a bit different because body shapes of various cars/trucks, how well it is grounded and were the antenna is mounted influence this.
Most guys installing only have a SWR meter and that is fine, a long as the meter is telling you that SWR is about 1.5:1 the average radio will be happy.
When you get deeper into antennas and start using a antenna analyzer and looking at things more closely there is more to it than just SWR.
The Coil he is using helps to bring the match closer to 50 Ohms at the feed-point.


73
Jeff
Click to expand...

It is strange, to me, that the SWR rose when I bonded the truck. I thought it would have the opposite effect. Shows what I know. But it was the same with the fishing truck, so I guess that is normal.
My SWR is different depending on where i am. (Radio only) Open field it is 1.3:1 with 30W DK and when I drop the power to 10W DK it barely moves the needle. In the driveway it is 1.4:1 at max power. My problem is that I want it 1.2 with my amp on. I just bought a cheap tuner (RF limited) and hope I can match the impedence better to get the SWR lower. I didn't tell the wife I bought it but it was only $40. I'm not supposed to buy anything else for a while. I just don't want to have to keep ripping down my headliner every time I need to mess with the antenna. The headliner is going to get damaged the more it goes in and out. Some of the clips are already getting weak.


BTW, thanks for the offer DB. I wish I did live closeby because I'd take you up on it. I could use some experienced help and proper testing equipment right about now.
 
Greatvalue1 said:
What is an inductance coil that he is talking about in this thread? And why use one on a 102" whip?

I thought these antennas were generally pretty well matched for 11 meters.
Click to expand...


I am still learning this so forgive me if I am miss something. I am sure someone will correct me if I'm wrong. The way I understand it is:
The physical length of the antenna is correct for 11 meters as the SWR curve is at its lowest on channel 19 of the CB band. When I began bonding the body of the truck (RF grounding) with ground strapping, it changed the size of the counterpoise. It is for the purpose of attempting to make the radiating pattern better and the antenna perform better on TX and RX. But changing the size of the ground plane causes a mismatch of sort. SWR is still lowest on ch 19 but was higher than before RF grounding. The physical length of the whip has not changed. The shunt coil "what I called an inductor" borrows capacitance from the ground plane (truck body) and gives it to the radiating element (whip) and bringing it back into balance, thus lowering the SWR.
 
There's a big difference between a 'loading' coil and an impedance 'matching' coil in both their placement and their size. A 'loading' coil provides the inductance reactance required to neutralize the over abundance of capacitance reactance of a 'too short' antenna has. When the combined inductive and capacitive reactances equal zero reactance the antenna is resonant. Adding length adds inductance, removing length reduces inductance which means increases capacitance. (Confused up yet?) Inductive reactance is (+), capacitive reactance is (-). Add the right amount of '+' to the right amount of '-' and you end up with '0', which is resonance. The amount of wire used to make that loading coil has absolutely no direct relation to how 'short' the antenna is. A length of straight wire has a certain amount of inductive reactance. If you coil up that length of wire, it will have a lot more inductive reactance than it did when it was straight. The resulting inductance produced has to take into account the diameter of the coil, the space between windings, and the size of the wire used. There's a formula for that but it is not a simple one. Look it up, see for your self.
A 'matching' coil is placed at the feed point of the antenna, the other end is connected to ground, or the braid of the coax. The size of that coil is quite a bit smaller than is a loading coil. The combination of the antenna in parallel with an inductor to ground adds enough inductive reactance to produce an input impedance of the desired amount (usually 50 ohms). [The 'tricky' part of that is that -impedance- is composed of resistance and reactance. Find a good AC electrical text to understand it, I won't even try to explain it. And that reactance comes in two kinds, '+' and '-'. Just keeps getting 'better' and 'better', huh?]
Where you find reactance, and how it's used, can make a huge difference. A loading coil and a matching coil are very nice examples of that. Clear as mud, right?
- 'Doc
 
That's the biggy, really. Resonance doesn't have a -set- impedance, so the SWR with a resonant antenna can be almost anything, very seldom, if ever, 50 ohms or close. There's -always- two parts/steps in tuning any antenna, obtaining resonance and getting the input impedance to where you want it. Sometimes you get lucky and the input impedance ends up close to what you want, so there's no need to adjust/change the input impedance. You can 'fudge' a little, get one of those two things close and make the other one what you want. But that means that the part you 'fudged' is then off a little (how tuning most antennas ends up). It works, but is never the 'best' it could be. But if it's "close enough" then... it's close enough, you know?
What's the big deal with resonance? Resonance means that there's no reactance present in an antenna, not that it's always a particular length (loaded antennas for instance). Reactance doesn't contribute to radiating anything, that's the biggy! If part of the signal getting to that antenna isn't radiated then it's wasted, might as well not have been produced to start with. So getting rid of reactance -IN- the antenna is good. It means that all the signal getting to that antenna is radiated.
There's another aspect about antenna lengths that's important. The shape of the radiation pattern is determined by antenna length. You may be radiating everything coming out of that transmitter, but if it ain't going where you want it to go, then 'they' won't hear you (or you them). That's why particular length antennas are desirable, they produce a radiation pattern that put's the signal where you want it and hears where you want to hear. A very rough way of thinking about antenna lengths is that the shorter they are the less range when compared to a 'full sized' antenna. There are limits to that, antennas can be too long to.
Ain't very 'simple', is it...
- 'Doc
 
When it comes to impedance and resonance think of it this way, impedance matching determines how well the energy transfers from the coax to the antenna, resonance matching determines how well the antenna uses the energy that was transferred to it.


The DB
 
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Your are in information over load with no way to sort it out.
You need to go the school to learn AC impedence theroy first then another course on antennas and transmission lines.
Then what you hear here will begin to make sense and be able to reject myths and hear-sey.
Until you do you will be in confusion.
For instance a flat SWR has no direct relation to antenna resonance.
Feed impedance has no direct relationship to antenna resonance.
For example take an off center fed dipole. Meaning one side is longer than the other.
Even if the antenna is resonant on 11 meters; if the feed point is moved along the antenna by changeing the feed point without changing the total length of the antenna, the only thing that changes is the impedance at that feed point, demenstrating the feed impedance has nothing to do with the antenna resonance. Only the impedance changes.
Why: the voltage and current along the antenna changes with postion or the same general reason the formula R= E/I. If either E or I or both change, R changes, not the resonant frequency.
You keep trying to connect them togather. You can't.
This is why school is needed to fully understand as you say you want to do.
Hope I have not insulted you but to just bring some of the the facts to light.
Good luck.
 
roadrage,
You are not by your self in that, "answer one question and another creeps up" thingy, we are all in the same boat. The thing is that some of us/them have just been there a little longer so don't have to make the same mistakes again (IF we're lucky). Everything's that way, right? Someone once said, "If you are in hell, keep going.", that makes a lot of sense. Unless you want to stay there...
- 'Doc

(That was Winston Churchill, BTW.)
 
KM3F said:
Your are in information over load with no way to sort it out.
You need to go the school to learn AC impedence theroy first then another course on antennas and transmission lines.
Then what you hear here will begin to make sense and be able to reject myths and hear-sey.
Until you do you will be in confusion.
For instance a flat SWR has no direct relation to antenna resonance.
Feed impedance has no direct relationship to antenna resonance.
For example take an off center fed dipole. Meaning one side is longer than the other.
Even if the antenna is resonant on 11 meters; if the feed point is moved along the antenna by changeing the feed point without changing the total length of the antenna, the only thing that changes is the impedance at that feed point, demenstrating the feed impedance has nothing to do with the antenna resonance. Only the impedance changes.
Why: the voltage and current along the antenna changes with postion or the same general reason the formula R= E/I. If either E or I or both change, R changes, not the resonant frequency.
You keep trying to connect them togather. You can't.
This is why school is needed to fully understand as you say you want to do.
Hope I have not insulted you but to just bring some of the the facts to light.
Good luck.
Click to expand...

I am beginning to understand but still have a lot of open questions. I get idea of the resonance of the antenna. The point where the most amount of energy transferred to the antenna is radiated out. I understand what SWR is and how it has less relation to SWR than most people think but still have relation. And how shortening antennas and adding loading coil lowers the bandwidth of an antenna. As an example, my K30 on the wife's car has an SWR of 1.2 on ch 19 but on ch 1 and 40 it is 1.6. The SWR curve is steeper than, say, my Wilson 1000 that had an SWR of 1.2 on ch 1 and 40 and didn't move the needle on 19. Though the Wilson still has the need of a loading coil, it has a 62" whip vs the 36" whip of the K30. The longer radiator on the Wilson makes it more resonant over more bandwidth. Then my 102" is very broad banded, being a full 1/4 wave, and not needing a loading coil. The SWR is flat across all 40 channels. I need to learn more of capacitance, reactance, capasitive reactance, inductance. My biggest problem is that knowing the antenna theory won't help too much as I have no way of measuring for any of it, which also means I can't fix it.
 
binrat said:
RR,
you need a mfj 259b or 269 to make things easier for you.
Click to expand...

I know. Trust me, I am not even bringing it up to my wife right now. She may just dig out the shotgun and shoot me. I didn't work last week at all. I keep thinking of selling my electric guitar and amp to buy one. I never seem to list it though. I haven't even picked it up in about 2 years. I do have a radio I'm not using but it wouldn't get me close to paying for an analyzer. Its just a tuned Cobra 29 WXSTNW, which may yeild $60-$70 on the high side. It, like most of my radios, was a present from her, so I couldn't sell it because of principle.
 
HAHAHA, sell the shotgun before she uses it on you. :D
Its real handy to have, I never use a swr meter anymore since I got a 259b a few years ago.
 
A radio shop in our local truck stop here will analyse an antenna system for nothing! Nice guys!
Wasn't the 102" SS whip designed for 23 channel radios?
 

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