102" Whip Antenna Analysis

[freecell]

if resonance did occur at 27.5, and i'm not sure because i also saw the figures 27.75 & 27.80 then another 1.25 inches in addition to the 105.9375 you previously mentioned would have put resonance very close to 27.185 @ 107.1875. within less than another 1.4" the 27.80 figure could also be corrected to 27.185. the use of the 246 formula here is extremely accurate.

 

[W5LZ]

QRN,
Basically, yes.
- 'Doc

Worrying about a tenth of an inch is sort of rediculous. Drive two doors down the street and it'll probably be just right...

 

[C2]

If I can, I may accomodate EDUK8TR and take a measurement while driving.

Resonance is where it is, for this setup, at 27.75 MHz @ 105.9375"

The 27.5 MHz is just where I put the marker and was not really relavant (not at the lowest SWR either, the little peak looked interesting though).

Lowest SWR of 1.09 actually occured at 27.41 MHz with R = 54.4, X = -1.5

 

[freecell]

"Resonance is where it is, for this setup, at 27.75 MHz @ 105.9375."

too short for resonance @ 27.185....thanks for clearing that up.

 

[C2]

LOL

Let me qualify that.

Drive two doors down the street and it'll probably be just right...

Your right, just a few feet and the SWR plot changed! Going down a residential street was interesting as the VSWR snaked up and down, basically following the same curve. I'm guessing it was the overhead power and telephone lines that line the street and cross to the houses.

I took measurements in the clear at 30 MPH, 60 MPH, 80 MPH, on top of a bridge at 60 MPH, right next to a big rig at 60 MPH, driving by a big power distribution plant under the high voltage lines (about 20 lines), and finally standing still in a big open parking lot with nothing around.

I'll have to post some plots later, but suffice it to say that the SWR curve shifted higher in frequency to about the middle of the ham band when moving. All else had little effect, and the effect was somewhat marginal anyway. I certainly proves that SWRs do not go off the chart when the whip bends back. You might notice a couple of 10ths rise, however.

I think the measurement in the parking lot is the best reference for this setup, since my first measurements had obstructions that were probably close enough to cause an effect.

 

[freecell]

the swr fluctuation is the result of constantly changing values of -jx as the distance between the vehicle ground mass and the antenna itself constantly increases and decreases as the whip is set into motion from the stop and go movement of the vehicle.

with the vehicle in steady forward motion and the whip leaning to the rear levels of -jx are present in large enough quantity to electrically shorten the antenna driving it up into the middle of the 10 meter amateur band. both swr and resonance are constantly changing in step with the motion of the whip.

 

[W5LZ]

C2,
Everything changes, resonance, impedance, reactances, color of your shirt, whatever. Depends entirely on the variables associated with the antenna, design, limberness, environmental, and who knows what else in any particular situation. Sometimes, I think the color makes a difference, like that shirt! (The only thing cast in stone was the ten commandments and somebody broke those - lol!) The 'best' that you can hope for is a good 'average' close to where you want it. Does it really make all that much difference? If the variables can be accounted for half way decently in such a manner as to remain in the 'practical' relm, no, it doesn't. That doesn't mean that a straightened out coat-hanger is just as good as a 'real live' antenna, but worrying excessively about it is a total waste of worry. So, there isn't any absolute, 'dead-nutz', always perfect, "magic" number for figuring antenna length. Over time that "magic" number can be refined to a sort of 'standard' one, which has been done (234, 468, 984, whatever). Then you modify the resulting length till it's as good as it gets in a particular situation (called 'tuning'). And staying in the 'practical' relm, it's better if that "magic" number is slightly too big than too little, cutting is easier than adding on. (If it were so @#$ easy, why would anyone need a genius electronic's technician to work on stuff... like us! Spread that on your garden for those 40 pound 'tators'.)
All of which says nothing about what/how/where/when you do the required measuring, and with what.
- 'Doc

PS - Over 100 here in the 'puter room. I'm going outside where it's cooler. Disregard any misspelling, obvious mistakes, etc, at this point I really don't care...
(Rat Shack ther-mo-meter but I believe it!)

 

[C2]

of course, Doc.

I think its the "magic" or standard number were looking at.

I should just read the ARRL Antenna book, but you know, the 234 number is what I have seen more often than not, and it seems to not be happening here. Of course, I would expect it to put one quite close the majority of the time.

Interesting that I was able to support a free space number instead. Maybe I'll change my coax length while I'm at it.

It's all good. I'm sure somebody is taking notes. At least I hope the data is more representative of facts than mere lip service.

Thanks for playing along guys...it's been a blast

 

[freecell]

"Over time that "magic" number can be refined to a sort of 'standard' one, which has been done (234,........"

"I think its the "magic" or standard number were looking at."

or you're both blind, deaf and dumb. or if you really believe that then adjust the length to 103.3" and go run some more test measurements and let's bring all of this to its inevitable conslusion. all that is going to happen is it will move further up into the 10 meter band, the same thing you saw when the whip was swaying to the rear of the car while it was in forward motion.

when that occurs with 103.3" you'll see it move a lot further up the band than it did previously. and somehow you still insist that the 234 formula is the one that you're working with here. it's not happening. if it was you would see resonance with the vehicle (and antenna) sitting still in the clear somewhere inside the citizens band at a length in the neighborhood of 103.3", per the formula that you seem to think applies here. how you can still believe that at this point with the information that we already have is beyond me.

the shorter the length of the antenna the higher the resonant operating frequency will be. the longer the length of the antenna the lower the resonant operating frequency will be, it doesn't get much simpler than that. if you have seen already that at just under 106" that resonance was occurring at approximately 27.75 then what gives you the impression that shortening it further to comply to the 234 formula is going to move the resonant condition further DOWN THE BAND? if anything it needs to be longer even if only to counteract the shortening effect that is created when the vehicle is in forward motion and at speed. if anyone is trying to make this "fit" into some formula it certainly isn't me. and i have news for you, it doesn't in the two cases discussed here and it won't.

here's the statement i made that started all of this:

the 102" steel whip in addition to the spring and ball mount produces a total length of approximately 108".

246/27.185 = 108"........that was my answer to excavator and it's still my answer, nothing presented here has changed a thing.

 

[C2]

you're blind, deaf and dumb.

If it makes you feel better, freecell.

But I think you are not understanding where I am comming from. I am saying that the constant that I have seen for calculating the 1/4 wavelength in a wire (or metal antenna in this case, simplified) is 234. I have seen it on web pages and have been told it is mentioned in the ARRL antenna book. I have also read that the number is based on the VF of metal.

I have not personally read what is said in the ARRL Antenna handbook about why 234, W5LZ says there is something in there.

The point is, I did not pull the 234 number out of my rectal cavity, it exists in other places.

I am not saying that I believe that using the 234 number in my setup is going to make my antenna resonant at a lower frequency than it is presently.

And I am not questioning my measurement.

What I said was, "the 234 number is what I have seen more often than not, and it seems to not be happening here."

Is this what you think makes me deaf, dumb, and blind?

I also said, "Of course, I would expect it to put one quite close the majority of the time."

Maybe that was it? How dumb of me to even mention the number?

whatever makes you happy and feel better about yourself

And if you think your so smart, maybe you can explain why that other site, the one you pirated the smith chart from, finds resonance of a dipole with a number more like 230.

or if you could do that, you could also explain why 234 does not seem to work here, or why it is mentioned in other places, rather than just a smart alek remark like just because, or go try it, or revert to some subordinating comment.

maybe there is some other context for the 234 number? maybe there is a reason for the 246 number beyond just because?

 

[freecell]

"maybe there is a reason for the 246 number beyond just because?"

just because resonance occurs at a given frequency when X = 0. lengthen the antenna and the point where X = 0 moves down the band. shorten the antenna and the point where X = 0 moves up the band.

in the 234 example under the installation conditons presented in both of the test antennas X does not equal 0 anywhere near 103.3" and X does not = 0 anywhere near the center of the band in your particular case until the length of the antenna is increased beyond 105.9375".

i have already explained several times why the 234 doesn't work here. here it is again. X does not = 0. i alluded to some of the detail in one or more of my recent posts. re-read them. if you want to see something closer to 103.3" try moving it to a corner of the vehicle and eliminating 270 degrees of the ground plane mass immediately underneath it and see what happens.

 

[C2]

why the 234 doesn't work here. here it is again. X does not = 0.

That is just a "because" answer.

eliminating 270 degrees of the ground plane mass immediately underneath it and see what happens.

One small step in the right direction. Thanks...

 

[freecell]

unfortunately that type of location doesn't conform to what is considered an ideal mounting location for an antenna, particularly if something closer to omnidirectional operation is desired.

 

[C2]

:idea: I think I got it. Thanks.

 

[freecell]

your "230" question is a no-brainer too.

the answer is right in front of you....i believe the frequency in question was 300.733 Mhz.. that should tell you all you need to know. look at the *"increasing diameter to length ratio" of any quarter wave antenna and realize as the operating frequency becomes higher and higher the antenna becomes shorter and shorter, so short that the *"increasing diameter to length ratio" introduces inductive reactance into the antenna making it electrically longer than indicated by its physical length when referenced to either of the two previously mentioned formula variations. i have no doubt that a 230 formula at this relatively high frequency with the usual materials and construction would compensate for this inductive loading effect*. the formula is constantly being corrected to compensate for this and other variables.