A question on whips...

[guitar_199]

Everywhere I go... I see people saying that when you use a 102" whip... you need to add a 6" spring to get the correct length of 108".

There has to be something I am not getting.

If one considers that a halfwave is 468/f in mhz.... then for the middle of the band... it would be 468/27.185 which is...

17.22 ft ... or ...... 206.58"

Divide that by 2 for a quarter wave and you get...

103.29".

If you turn the math around....

A 108" whip .... twice....is.

216"..... or..... 18 ft exactly.

468/18 would be 26 mhz.

Why is it a popular opinion that the total length of the whip should be 108"?

What am I missing?

Thanks!
Bob

 

[Captain Kilowatt]

I never ran a spring with my fibreglass whip. I ran it on both 10m and 11m and found that the SWR rose near the bottom of the CB band but held quite well up into the middle of 10m. Using a spring, it would have not been possible to cover both bands.

 

[The DB]

Why is it a popular opinion that the total length of the whip should be 108"?

What am I missing?

Some people round the half wavelength length to 18 feet because it is easier, others use that length because they don't know any better.

Also, to expand on what you noticed, if you take an antenna and move it around the chassis of a vehicle and measure it, you will notice that the tuned length also changes. This is because the chassis of the car also affects this. There is no one tuned length that works on every vehicle, or even every mounting location on the same vehicle. In my time, I've seen tuned lengths for these stretch from longer than 105 inches and shrink to less than 100 inches.

An easy way to see this in effect is to use a magnet mount antenna. The same thing that happens to a longer whip also happens to a magnet mount antenna...


The DB

 

[groundwire]

Everywhere I go... I see people saying that when you use a 102" whip... you need to add a 6" spring to get the correct length of 108".

There has to be something I am not getting.

If one considers that a halfwave is 468/f in mhz.... then for the middle of the band... it would be 468/27.185 which is...

17.22 ft ... or ...... 206.58"

Divide that by 2 for a quarter wave and you get...

103.29".

If you turn the math around....

A 108" whip .... twice....is.

216"..... or..... 18 ft exactly.

468/18 would be 26 mhz.

Why is it a popular opinion that the total length of the whip should be 108"?

What am I missing?

Thanks!
Bob

Also if i remember correctly, that antenna formula gives results for antennas in free space. so with that in mind there will be real world variables as far as antenna resonant length.

 

[Handy Andy]

You're getting a lot of good experiences, if not great experiences with 108" versus the 102".

But to say, "Why this?" Look at the mounting locations for many of the users of the 102" - by default, many have to use the shock spring so it could survive the trips the antenna gets taken along for the ride.

Others use such, so the SWR doesn't get too goofy when they drive and the antenna "peels" back in the whack of wind making the SWR will jump around as well.

A shock spring is just not another pretty face...

IT has a braid strap (or REAL ones should) that allow the RF into the antenna whip and not lollygag thru the coil winds -. affecting the SWR to a much higher degree,

So, if the SWR seems tolerable - then by all means you don't need the shock spring - unless you hit overhead objects and that is where the real problems begin...using nothing to absorb the shock of the strike, the whip can snap off.

Some are forced to use a shock spring of some sort just so the antenna lives to see another day.

 

[freecell]

an antenna is in free space when the real earth ground beneath the elevated antenna has no affect or influence over the manner in which the antenna operates. using the free space formula for calculating antenna length for antennas that are not in free space will result in resonant operating frequencies higher than your intended operating frequency.

as the distance between real earth ground and the antenna decreases, increased capacitive coupling between the earth ground and the antenna decreases the electrical length of the antenna, driving the resonant operating frequency higher as a result.

the formula for determining the resonant length of an antenna over real earth, as in the case of the popular 1/4λ steel whip, the correct formula for the center of the cb band is:

246 / 27.185 = 9.0491079639507081110906750045981 feet
246 / 9.0491079639507081110906750045981 = 27.185
27.185 X 9.0491079639507081110906750045981 = 246
108.589 inches

the hd ball and spring are required for resonance in the cb band.

for 1/2λ center fed dipoles

492 / 27.185 = 18.098215927901416222181350009196 feet
492 / 18.098215927901416222181350009196 = 27.185 mhz.
27.185 X 18.098215927901416222181350009196 =
491.99999999999999999999999999999
217.178 inches

all measurements in inches can be divided by 39.37 for conversion to meters.

when the feedpoint of the antenna approaches heights of 1.5λ above real earth ground and beyond then you can use the free space formula. @ an operating frequency of 27.185 mhz.. this would require a minimum feedpoint height of 54.294 feet.

 

[guitar_199]

Thank you for this... and I think that it makes sense. What I take away from your message is that... ON A VEHICLE....as you move the antenna around.... the placement on the vehicle is going to affect the "ground plane" more intensely... and you will have to tune the length to compensate for the resultant SWR change.

I just remember "two trucks ago".... cutting a hole in the left rear wheel/fender area for a ball mount (it was a Flareside pickup!) and putting a simple 102" whip with no spring... and my SWR was under 1.1:1 for the entire 40 channel band. Didn't have to play around at all.

Thank you!

Some people round the half wavelength length to 18 feet because it is easier, others use that length because they don't know any better.

Also, to expand on what you noticed, if you take an antenna and move it around the chassis of a vehicle and measure it, you will notice that the tuned length also changes. This is because the chassis of the car also affects this. There is no one tuned length that works on every vehicle, or even every mounting location on the same vehicle. In my time, I've seen tuned lengths for these stretch from longer than 105 inches and shrink to less than 100 inches.

An easy way to see this in effect is to use a magnet mount antenna. The same thing that happens to a longer whip also happens to a magnet mount antenna...


The DB

 

[freecell]

"putting a simple 102" whip with no spring... and my SWR was under 1.1:1 for the entire 40 channel band"

you may have measured a 1.1:1 swr but the antenna was not resonant @ any frequency inside the cb band.

246 / 8.5 feet = 28.941176470588235294117647058824 mhz..
246 / 9.049 feet = 27.18532434523151729472869930379 mhz..

 

[guitar_199]

Thank you for this! This actually ties a couple of things together for me.... I hope this makes sense..... as I describe it.....

I have always heard something about... accounting for the velocity factor in metal (stainless, copper, etc....) but shortening by 5%.

So when I look at the REAL vs FREE SPACE formulas that you presented....

246 for a quarter wave (double it would be 492 for a halfwave)

now take that 492 x 0.95 (shorten by 5%) and you get.... 467.4 (which is close enough to 468 for me).

Now maybe I am tying the wrong things together.... but it seems to make the numbers work out!

But I do get the idea... that lowering the feed point is going to have that effect so thank you for that. I will take this concept and keep it in mind as I go!

Thanks!
Bob

an antenna is in free space when the real earth ground beneath the elevated antenna has no affect or influence over the manner in which the antenna operates. using the free space formula for calculating antenna length for antennas that are not in free space will result in resonant operating frequencies higher than your intended operating frequency.

as the distance between real earth ground and the antenna decreases, increased capacitive coupling between the earth ground and the antenna decreases the electrical length of the antenna, driving the resonant operating frequency higher as a result.

the formula for determining the resonant length of an antenna over real earth, as in the case of the popular 1/4λ steel whip, the correct formula for the center of the cb band is:

246 / 27.185 = 9.0491079639507081110906750045981 feet
246 / 9.0491079639507081110906750045981 = 27.185
27.185 X 9.0491079639507081110906750045981 = 246
108.589 inches

for 1/2λ center fed dipoles

492 / 27.185 = 18.098215927901416222181350009196 feet
492 / 18.098215927901416222181350009196 = 27.185 mhz.
27.185 X 18.098215927901416222181350009196 =
491.99999999999999999999999999999
217.178 inches

all measurements in inches can be divided by 39.37 for conversion to meters.

when the feedpoint of the antenna approaches heights of 1.5λ above real earth ground and beyond then you can use the free space formula. @ an operating frequency of 27.185 mhz.. this would require a minimum feedpoint height of 54.294 feet.

 

[freecell]

it's easier to shorten an antenna that is too long than it is to lengthen an antenna that is too short. when you can make it through an entire post on this particular subject without mentioning swr then you're catching on.

you're welcome.

 

[guitar_199]

Tell me this.... can an "antenna analyzer" of any kind actually SEE resonance?
Like....one of the MFJs? Or a NanoVNA? Or are you suggesting more...... just "faithfully set it to the right length and forget about SWR?"

really am trying to get a grip on this idea!

[edited to add]
an additional question....
if one has an antenna analyzer...and tries to set it for "proper impedance" and favoring THAT ...OVER SWR..... is that a reasonable way to look at it?

"putting a simple 102" whip with no spring... and my SWR was under 1.1:1 for the entire 40 channel band"

you may have measured a 1.1:1 swr but the antenna was not resonant @ any frequency inside the cb band.

246 / 8.5 feet = 28.941176470588235294117647058824 mhz..
246 / 9.049 feet = 27.18532434523151729472869930379 mhz..

 

[freecell]

"can an "antenna analyzer" of any kind actually SEE resonance?"
yes, most of them are capable of performing that function.

there is no favoring either impedance over swr or visa versa because you can't have one without the other. for example, @ an swr of 1.5:1 there are only two purely resistive (non-reactive) impedances relative to a 50 ohm system that would result in a 1.5:1 swr, either 33.33 or 75 ohms. without current and voltage there is no impedance and without impedance there is no swr measurement. in regard as to the resonant frequency of the antenna, if there is any reactance present in this scenario then the load or antenna is not purely resistive and values will be different than 33.33 or 75 ohms. uncancelled reactance in the antenna will increase the swr on the feedline. remember, the swr on the feedline is determined only by the matching conditions @ the load in the case of reflectionless matching performed at the (load) antenna.

 

[undertaker]

Must admit my 102 whip with the spring works great on the back of my toolbox it is in the center.

 

[Captain Kilowatt]

Also if i remember correctly, that antenna formula gives results for antennas in free space. so with that in mind there will be real world variables as far as antenna resonant length.

Actually using 468/F takes free space out of it, however all installations are different so individual mounting locations will give different results. IIRC free space uses 1005/F to determine a full wavelength therefore it would be 502.5/F for a half wavelength. Since neither you nor I live in free space, we should just stick with the 468/F and live with it.

 

[freecell]

1005 ÷ fmhz. is the formula for a full wave driven loop element, not a straight full wave element. the formula for a full wave loop reflector element is 1030 ÷ fmhz.. 492 ÷ fmhz. is the formula for a center-fed 1/2λ dipole or an end fed 1/2λ vertical over real earth ground. as mentioned by groundwire earlier, 492 X 0.95 = 467.4 or 468 which is the free space formula conversion from the over real earth ground formula.

an antenna is in free space when the real earth ground beneath the elevated antenna has no affect or influence over the manner in which the antenna operates. using the free space formula for calculating antenna length for antennas that are not in free space will result in resonant operating frequencies higher than your intended operating frequency.

as the distance between real earth ground and the antenna decreases, increased capacitive coupling between the earth ground and the antenna decreases the electrical length of the antenna, driving the resonant operating frequency higher as a result.

capacitance induced or coupled into an antenna electrically shortens the antenna and raises the operating frequency.

inductance induced or coupled into an antenna electrically lengthens the antenna and lowers the operating frequency.

when the antenna is adjusted for (X) reactance = 0 @ any given frequenc/y/ies then all formula and velocity factor error is eliminated. these errors are generated by the λ to diameter ratio of the conductors used, i.e., wire vs. tubing, end effect and proximity to real earth ground.

cut any dipole for a given frequency using the 468 formula, place the feedpoint up @ 1/4λ above real earth ground and then using an analyzer, scan the band until you find the frequenc/y/ies where X = 0 and compare that to the frequency that you originally cut the dipole to operate at.

YDRC

maybe this will help: https://duckduckgo.com/?q=492+÷+frequency+in+mhz.+=+length+in+feet++&t=h_&ia=web
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