Different Look At 102" Whip Discussion

[AudioShockwav]

I do not post much on 1/4 wave threads because I often get myself in trouble with this topic because of my choice of words, but....


Posted By WW:

The math will show that the center resonance point on 102" is right about at 27.520 mhz or "Channel 52" for you sophisticated raydideo operators.

Posted By BM:

the ground plane will effect the final tuned length .

Posted by 6 Liter :

that length would probably be more in the instance of a poor groundplane? With a better performing ground plane, the vertical would be a bit shorter?

Posted by Marconi:

I might agree with WW regarding the 102" whip installed on a mobile, but it is not specifically a problem with the length of the whip just to be accurate. The problem with mobiles is a typical lack of a suitable ground plane for the radiator to work against.

You will see that this 102" whip and a good ground plane works very well in the CB band.

Posted By DOC :

SWR only tells you about the impedance match of the antenna system,

Posted By BM:

but take it off and put on a 102 whip and it shoots above 2.5 .

Posted By Marconi :

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.

Posted By Gamegetter:

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

I would Keep going, But there seems to be a pattern here.

This Post I agree with 100%

Posted By gamegetter.:

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

I know the example I am about to talk about is a Base antenna, but just to think about, ....the I10-K is tuned by setting the length of the antenna and THEN adjusting the trombone for best bandwidth/SWR.
( then re-read the above and take out the word trombone and insert the words feed-point for just a second)

Now Back to this:
Amateur Quarter Wave Ground Plane Antenna Calculator
The numbers are there, no mater how many times you try it.

And what DOC is saying about
""just because it shows good SWR don`t mean that the antenna is resonate""
many antennas can be the correct length, but exhibit SWR that is real bad, because of several factors.
Design
Feed
Location
Lack of Good Ground plane ( iF the design needs one)

Then Look at this Image From The K0BG. com web site.

many Guy`s, look at that and go...
W T F ?
That is BS....How can that work ??? You just shorted the Coax to ground!!!
That is a dead short!

I will finish...
An antenna Can show Good SWR and yet NOT be the correct Length.
An Antenna CAN be the correct Length and show BAD SWR.
This is why we need to MATCH the feed point of the antenna.
regardless of the SIMPLE idea of a 1/4 wave mobile set-up, it can often be complicated because there are so many factors that are different in a mobile mounting set-up.

RF is a Strange thing, Well...No RF is a Complex thing......
R= X= Ohms.......
SWR is but one thing in the whole picture, and in most cases it will get you by, because as long as you radio is happy with what it see`s at the coax connector on the back of the radio, it has that warm fuzzy everything is good feeling.
And "most" of the time that works well.
73
Jeff

 

[hookedon6]

Then Look at this Image From The K0BG. com web site.

many Guy`s, look at that and go...
W T F ?
That is BS....How can that work ??? You just shorted the Coax to ground!!!
That is a dead short!
73
Jeff

also notice the size/ diameter of the coil,.... the diameter is the same as the length, also called a "1x1" coil....... guess what happens when some golden screwdriver streaches the coil

 

[northern35s]

also notice the size/ diameter of the coil,.... the diameter is the same as the length, also called a "1x1" coil....... guess what happens when some golden screwdriver streaches the coil

The length and the width are as long and as wide as they need to be in order to match to 50 ohm, stretching the coil will alter the impedance by a small amount. My shunt coil is longer than it is wide, the reason? simple, the round tube I had to hand wasn't vary wide, it meant I had to add more coils to get the correct impedance, around 8uH as it happens

Shunt feeding a mobile antenna is usually required with a sub 1/4WL antenna, where the efficiency and Rr of the antenna, at resonance, will be low, especially on the lower HF bands.

A less than perfect ground plane is likely to increase loss, and as crazy as it may sound, your feedpoint VSWR may look pretty good as a result, however you're unlikely to notice this with a VSWR meter, that is the domain of the antenna analyser.

 

[jazzsinger]

Velocity factor of a 1/4 wave steel whip in air which is the dielectric (the main determining factor of propagation velocity, but type of conductor has effects too) ,lies between .95 and .99 of a wavelength. i ain't getting into that debate.

so here goes

at 100% velocity factor, 1 wavelength at 27.205 mhz = :

299792.458 (exact speed of light) / 27205.000 (exact frequency, unless your using a cb,where it will be as close as dammit) = 11.02m or 11.02 x 3.28 (converts from metres to feet, if you want metres from feet divide by 3.28) = 36.14feet
(or for you hardcore imperial guys 36 feet 1 3/4 inches approx.)

.95vf = 299792.458 / 27.205.000 = 11.02m or 36' 1 3/4" x .95 = 10.47m or 34 feet 4 inches approx. divide that by 4 for a 1/4 wave and you get 2.62m or 8 feet 7 inches give or take a few thou. x12 to give inches = 103.01inches, or 103 inches and a thou or 2.

.96vf = 299792.458 / 27.205.000 = 11.02m or 36' 1 3/4" x.96 = 10.57m or 34 feet 7 inches (approx) /4 = 2.64m or 8' 8"(roughly) x 12 = 104 inches.

.97vf = 299792.458 / 27.205.000 = 11.02m or 36' 1 3/4" x .97 = 10.68m or 35 feet 1/2" give or take a thou or two, /4 = 2.67m or 8' 9" x 12 = 105 inches or so.

.98vf 299792.458 / 27.205.000 = 11.02m or 36' 1 3/4" x .98 = 10.79m or 35 feet 3 3/4 inches or so. /4 = 2.69m = 8 ft 10" x 12 = 106 1/4 inches

so there you have it, no fancy antenna calculators, just a calculator as found on any windows pc and a bit of knowledge and you can instantly see that no matter what people agree the velocity factor of steel is in air, its not going to resonate on 27.205 Mhz if its only 102" Long

i ain't getting involved in vf of steel conductors in air, but my personal opinion is it lies around .97-.98 .


If i recall correctly US steel whips were originally 108",and this 102 inch whip notion came from the fact an antenna that has a fibreglasss coating as the dielectric and not bare, too the air, is where 102 inch whips originally came from.Which would indeed shorten the antenna for resonance, as fibreglass will have a different vf constant to air.

i may be wrong here but i believe 102 inch steel whips were originally designed for uk market which has a higher centre frequency than fcc frequency, and 96 inch whips came from coating uk designed antennas in fibreglass.

it may be sheer coincidence, but i doubt it, that fibreglass coated whips for uk market are 6 inches shorter than the steel ones, and by sheer coincidence 102 inch fibreglass whips are 6 inches shorter than the US steel ones, that used to be sold at 108" , is that sheer coincidence?

Draw your own conclusions.

 

[The DB]

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

There is a free .pdf file out there called "Another look at Reflections" by M. Walter Maxwell that has a two sections called "Part 5 - Low SWR for the Wrong Reasons" and "Part 6 - Low SWR for the Wrong Reasons (continued)".

Is it possible to have "to low" of an SWR? I suppose not. That being said, just because you have a low SWR does not mean that your SWR is low for the right reasons. It also does not mean that the antenna is tuned to radiate as well as it can for the frequency you are using.

SWR to me is an indicator, nothing more and nothing less. If the antenna I'm working with should have an SWR of 2:1, then 2:1 is what I want, even over the supposedly "perfect" 1:1 reading.

There is far more to properly tuning an antenna than the highly mis-understood and overrated SWR.


The DB

 

[Lil'Yeshua]

I thought it was 8.66ft.(For the freq I'm using). I got four 102's at home. One up on the tower and three new Rat Shack ones still in their tubes. I think I'll go measure one to see if it's really 101" like Cryptic Cricket says they are.
I was going to use them for my driven elements for my CP antenna before I got my CP antenna plans. I've grown a lot smarter since then.


P.S. I always tune for the right RF output which doesn't coincide with the lowest SWR.

 

[Marconi]

DB, how do you tell what the SWR for a particular antenna is supposed to be?

 

[jazzsinger]

DB, how do you tell what the SWR for a particular antenna is supposed to be?

Eddie,
the best way to tell what the swr of an antenna is supposed to be is to see what type of connector the radio uses and what coax is recommended, thats what it is supposed to be the same as, but in real life and the real world that's rarely if ever the case,especially if you want it to cover more than one frequency.

the swr is a consequence of the antenna design, for a cb they are all supposed to be 50 ohms, but in reality only one i know off, the starduster type will give you that straight off from its design, others will give you it from their design plus matching devices, some just dont need it to be perfect 1.0:1 to work well, and trying to match antennas that are close as dammit to 1.0:1 from their design is pointless, much better to tune it for resonance and accept a slightly higher swr reading imo, 1/4 wave whips and 1/2 wave dipoles are two classic examples, although if you want to be really pernickity you can alter design slightly or add reactance of one type to neutralise reactance of another type, be it capacitive or inductive. some people who get a perfect 1.0:1 match on an antenna thats not meant to be 50 ohms have introduced reactance therefore mistuning resonance for their particular desired frequency, the majority get it from using shitty lossy coax that gives lower readings due to cable losses and fuck all else,

it never ceases to amaze me the look on peoples faces when i install better coax like rg214/u and not only does their swr go up, so does their performance and their noise generally drops too. i've even known some dumbasses undo what i did and go back to their old shitey signal just to see 1.0:1 swr. then they have the cheek to wonder how i can talk to the world on an hp 4000 with mil spec coax that shows true swr and i couldn't give a fuck if its below 3.0:1, cause its too fucking cold here to get oot the car to adjust it for a fucking minimal gain noone will ever notice except ma fucking frostbitten fingers.

 

[jazzsinger]

Amateur Quarter Wave Ground Plane Antenna Calculator

i'd be wary of anyones calculator who suggests you need to do two calculations to get the proper tuning on an antenna used over a wider band,

why not just use one calculation for the centre frequency?

it makes no sense whatsoever.double the work, double the confusion for a simple fucking task.

 

[The DB]

DB, how do you tell what the SWR for a particular antenna is supposed to be?

Some antenna impedances are well known, such as the quarter wave whip (with horizontal radials) and a half wave dipole. Others you have to do more research on.

Typically most antennas I see are quarter wavelength or shortened quarter wavelength antennas, which will give you an impedance at resonance between 10 and 36 ohms, which is between 5:1 and 1.38:1 SWR. An exception here is some base loaded antennas will use a capacitor along with the base load to try and better match to the 50 ohm feedline at resonance.

Many base antennas you can adjust resonance and impedance separately, those and the quarter wavelength verticals with angled radials are typically the antennas that can be tuned for both resonance and impedance at the same time and present a perfect 1:1 match for the right reasons.

I am under the impression that you could use modeling to make the determinations as well, I bring this up because you are a modeler. Unfortunately I am not, at least, not yet...


The DB

 

[The DB]

To further my above post a quote from M. Walter Maxwell's "Another Look at Reflections" .pdf file, page 39. This .pdf file is freely available on the web.

With an adequate ground system, the SWR is predictable over the frequency range, because a load impedance of any specific R + jX value yields an exact SWR on a given feed line, and because we know approximately what the antenna impedance should be at whatever frequency we may wish to use (refs. 4 7, 48, 49, and 58, p. 3-1). But when the ground system is inadequate there is an unknown ground-loss resistance added to the known antenna impedance, which changes the SWR to some lower, unpredictable value. Yet, without being aware of these facts, we often tend to be happier in the discovery of an unsubstantiated low SWR than we do in determining whether we have SWR values that should be obtained with the existing configuration. This is a very important concept that requires a clear understanding if we are to avoid misinterpretation of SWR data in our effort to optimize radiated power.

The DB

 

[Lil'Yeshua]

Survey says!
Note: this is a Radio Shack brand 102" SS 1/4 wave whip antenna.
Obviously Crytic Cricket has some other brand antenna.
This just comes show you to always double check things.
8.66 or so feet.
Note: These R/S whips come packaged in individual cardboard tubes with the ends sealed with aluminum end plugs

 

[Mudfoot]

RS used to have whips leaning in a corner. They were always different lengths, never consistent. I doubt they've improved. I have three Wilson stingers, all three are different lengths.

 

[Marconi]

Survey says!
Note: this is a Radio Shack brand 102" SS 1/4 wave whip antenna.
Obviously Crytic Cricket has some other brand antenna.
This just comes show you to always double check things.
8.66 or so feet.

I have six 102" whips and they're all close to 102.5" long to the bottom of the base, not including the threads.

Here is my Marconi 3x with 3 x 102" radials, and 1 x 102" radiator on a GPK hub from an A99, using a mirror mount L bracket, at 31' feet to the hub. There is an image of this setup in my profile image above.

View attachment Marconi 3x slanted.pdf

The black circles are reports noted from my Autek VA1 thru the feed line noted in the report details. The squares are reports from my SWR meter in line at the radio. Take note of the nice control over the complex (Z) values accross the very nice bandwidth <2.0:1. The SWR bandwidth also reflects a nice flat match as well.

 

[towerdog]

I dont think a 1/4 whip is often the best option for a CB antenna. Mounting it below metal, like on a bumper screws up radiation and swr. When driving 70 mpg and the top of it is leaning horizontal or bent like a u, i think thats a problem as well.

In theory a "102" whip would be better, and if you could mount it center of the roof, and find a whip stiff enough not to flop so much in the wind it would be better..but who the hell wants to put a hole in their roof besides me.

I would take a good base or center loaded whip on the roof over a "102" on the side or bumper.

I run a "102" on my truck, with 4 or 5 inch spring, mounted just forward of the tailgate, on the side of the bed. It just worked, had better luck with a K40 mag mount. SWR was not too bad for the lack of counterpose, about 1.3:1 on commercial grade meter. 1.3:1 at about 26.5mhz, more like 1.5:1 at 27.2, i just never got around to cutting it down as i dont really drive the truck anymore.

They need the spring on CB, and then cut down to work right.