102 in whip

[AudioShockwav]

Are these numbers assuming a V.F. of .95%?

( sorry I missed this somehow)

No, it is a straight calculation.
The thing is that a proper 1/4 wave antenna, installed over a good ground plane will result is somewhere around a 1.5:1 SWR.
This is because the feed point impedance in not quite 50 ohm`30/35/40 or so and things such as how much ground plane you have under it or what is around it effect it or how sensitive your meter is and so on.
So then comes the question i see asked the most " what length coax do i need to use to tune a 102 " whip?"
The real answer is; the length of coax it takes to reach the transmitter.
You really do not need to "tune " a 102" whip, you need to match the feed point impedance.
The usual answer is" stick a 3 or 4 or 6 inch spring under it.
This will dink the feed point impedance up closer to 50 ohms ....the SWR meter drops and everyone is happy.....except the antenna....that is now resonant down around 26.5 Mhz or so depending on how much you ad to the length of the antenna.
Will it work? sure it will, but it could be better.
Some one the other day said that lowest SWR and antenna resonance do not always occur at the same point, and I think that is where Doc was headed before his daily dose of coffee ran down this is very true.
The coil in the picture above "looks" to be a direct short to ground, and it is......to DC. But RF is a different beast, that coil is now an inductor, and it is helping to match the feed point impedance of the antenna.
Here is a good example.
Any of you guys remember the 1/4 wave "droopy drawers" base antennas?
1/4 wave vertical, with 3 or 4 1/4 wave radials.
The reason they "droop" the 4 radials, is that by pulling them down to about a 45 degree angle, you raise the feed point impedance to 50 ohms with out using a matching network of some kind at the base of the antenna..

73
Jeff

 

[gamegetter]

greetings

This has been a very informational thread and I appreciate it.

If at 27.205 mhz, resonance is at 103.216" for a 1/4 wl, then why would antenna manufacturers cut them at 102"?

 

[AudioShockwav]

There has to be some kind of mount under it, that will add to the length a bit.
Most of the time they get a spring stuck under them as well.
1.5:1 is fine for most setups.
for the Average guy installing these antennas, all they have is a SWR meter anyway, and a full length antenna is going to work reasonably well.


73
Jeff

 

[AudioShockwav]

Often you hear some one talking about antennas, and they start to compare loaded antennas with a full size vertical.
The common explanation is that a loaded antenna uses a coil to make up for the length that you do not have, so they will get this picture in there head of a full length piece of wire, taking it and winding some of it around some kind of fiberglass stick,and bang, there you go.
But if you take that mental picture apart, and string the wire out, it is not the length that you thought it would be.

There are other factors involved in building antennas
Here are some things to think about.
( I am going to use some simple examples of antennas, and how the work here, the idea is to get everyone to THINK about how antennas work and why, not pure techno-babble, but things to inspire thought)

#1
The A-99, or Antron 99.
The antenna is set at a fixed length, and if it needs to be adjusted, you screw two rings up and down.....now we know if we are tuning a mobile antenna, we have to move the stinger up and down to adjust it right?
So how can you tune a Antron without making it longer or shorter?

There is a Coil inside the fiberglass, and the metal rings, being close to the coil have an influence on how RF "see`s" that coil.
Coils have a property called Inductance,
and you can vari that by screwing the rings up and down, thus changing the "tuning" of the antenna.

#2
There used to be a line of fiberglass antennas, like a fire stick, helically wound, wrapped in plastic shrink tubing.
Near the top of the antenna there was a thick rubber tube, about 6 or so inches long.
So, again, from tunning mobile antennas, we know that we need to change the length of the antenna, so....you peel some of the tubing away and start trimming the end to adjust the antenna, right?
the answer, for this antenna is NO!
You simply slide the thick rubber tube up or down to tune the antenna!
But how can that work?
You are not changing the length of the wire????? so how can it change the SWR? That thick rubber tube surrounding the coiled copper wire at the top of the antenna changes the velocity factor of the wire in that section of the coiled wire, and effects the inductance of the "coil" of wire wrapped around the antenna, and allows you to tune it.

#3
Common Question.
Why is a Fiberglass 1/4 wave usually 96" and a stainless steel whip 102?
The wire in a fiberglass is surrounded by ----fiberglass.
The fiberglass changes the velocity factor of the wire and the whip is shorter because of that.

There is more to a antenna than just a chunk of wire.
Inductance
Reactance
Feed point impedance
Counterpoise
Ground losses
The "Q" of the Coil if it uses one.
A SWR meter is a good, simple tool, but a Antenna Analyzer can tell you the SWR and more...and help you understand more about what is going on in antennas.
I am not a expert on antennas, but I understand them to a point just because I like to play radio.
I try to keep a open mind about radio stuff, I like to learn and understand why.
I really encourage every one to use the internet to read, and learn as much as you can about antennas.
It will allow you to understand how and why they work, and in the end, allow you to enjoy playing radio even more.
The Web site I listed above, K0BG's site is very good and loaded with great stuff about mobile antennas.
Have fun guys.

73
Jeff

 

[gamegetter]

thanks

Having read this thread and portions of KOBG's web site, I have a little better understanding of antennas. I really appreciate folks like yourself and others who are patient to teach the rest of us who joined the forum to learn. We all learn at different paces and I know sometimes it gets frustrating for those who have alot of knowledge to hopefully share.

My main take away from this has been on learning about resonance and matching of antennas.

Audio, in your example of the A-99, I am assuming that the antenna is already resonant and that by adjusting the rings we are actually matching impedence. Is that correct?

If so, then I believe this should be the procedure to "match" all antennas mobile and fixed. In other words one should always begin with a resonant length antenna because this gives maximum efficiency of radiation and then the impedences should be matched.

This may be above most folks comprehension. Mine included, and I have been into radio off and on since the 70's and here I am now just learning this. Hence, I believe why antenna manufacuturers mainly focus on matching impedence by swr...however cutting or changing lengths can actually decrease tune or resonance and therefore efficiency.

thanks for the opportunity to discuss and to learn.

 

[W5LZ]

Just as a point of interest...

That 102" whip has been around a lot longer than CB radio has. It is used as a starting point in several different services, and is a 'reasonable' starting point from about 27 Mhz to as high as you think you might want to go. (I think at one time, every state trooper in the world had one on their car (35 - 50 Mhz).)
- 'Doc

 

[AudioShockwav]

(I think at one time, every state trooper in the world had one on their car (35 - 50 Mhz)

I think you can still see them out here on Cal Trans Trucks among other things,
Good point Doc.

73
Jeff

 

[AudioShockwav]

Audio, in your example of the A-99, I am assuming that the antenna is already resonant and that by adjusting the rings we are actually matching impedence. Is that correct?

Um...well I might have picked a better example, the A-99 is kind of in a class of it`s own, having two coils, a nylon spacer, and a short length of brass tubing the forms a capacitor, the rings change the inductance of the outer coil, but the length is fixed.
I was not going that deep into the matching network other than to use it as an example of a fixed length antenna.
If you want to dive into the a-99 there is a web site somewhere that shows the antenna all cut apart.


73
Jeff

 

[mackmobile43]

Just as a point of interest...

That 102" whip has been around a lot longer than CB radio has. It is used as a starting point in several different services, and is a 'reasonable' starting point from about 27 Mhz to as high as you think you might want to go. (I think at one time, every state trooper in the world had one on their car (35 - 50 Mhz).)
- 'Doc

Nebraska state and State troopers still use a steel whip.

 

[W5LZ]

"I am assuming that the antenna is already resonant and that by adjusting the rings we are actually matching impedence. Is that correct?"

Yes, it is. That antenna is resonant -somewhere-, but it may not be too close to where you want it resonant. (And why 'they' tell you how to cut the thing for resonance.) The 'rings'/matching section is exactly that, it matches impedances to 50 ohms (or close), it has very little/nothing to do with resonance. One reason that particular type of matching system is used is that it contributes a fair amount of reactance which seems to make the thing 'broader-banded' than a resonant antenna actually is. (Sort of like a 'bazooka' antenna's impedance matching system.)
SWR has little to do with any antenna's efficiency in radiating a signal (or receiving one). SWR along with resonance can make a very noticeable difference in efficiency! Of the two, resonance is more important I think (or in my 'opinion', if you prefer). The 'catch' in that is that a resonant antenna will NOT be very broad-banded. It just isn't gonna 'stretch' from 'day light to dark', which is also absolutely normal.
- 'Doc