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G4ILO
02-16-2009, 05:43 PM
I always thought it was 19 inches. That's what the formulas say. But when I put one of those 19 inch telescopic whips on my FT-817 I got three bars of SWR. By clipping some wire to the end I got it down to no bars with a total length of just over 22 inches. And not only that, the strength of the repeater I was monitoring came up.

I have checked this out with an external SWR meter and an antenna analyzer, and 22 - 23 inches definitely seems to give a better match. So why do they make these telescopic whips 19 inches max?

KL7AJ
02-16-2009, 05:45 PM
I always thought it was 19 inches. That's what the formulas say. But when I put one of those 19 inch telescopic whips on my FT-817 I got three bars of SWR. By clipping some wire to the end I got it down to no bars with a total length of just over 22 inches. And not only that, the strength of the repeater I was monitoring came up.

I have checked this out with an external SWR meter and an antenna analyzer, and 22 - 23 inches definitely seems to give a better match. So why do they make these telescopic whips 19 inches max?

Precisely one cubit

WB2WIK
02-16-2009, 05:46 PM
Precisely one cubit

Only if measured in footlongs per fortnight.:)

K7MH
02-16-2009, 05:52 PM
Precisely one cubit

6 palms.
24 digits.
It's all in my grade school Pee Chee folder...top right.

AK7EE
02-16-2009, 05:59 PM
I always thought it was 19 inches. That's what the formulas say. But when I put one of those 19 inch telescopic whips on my FT-817 I got three bars of SWR. By clipping some wire to the end I got it down to no bars with a total length of just over 22 inches. And not only that, the strength of the repeater I was monitoring came up.

I have checked this out with an external SWR meter and an antenna analyzer, and 22 - 23 inches definitely seems to give a better match. So why do they make these telescopic whips 19 inches max?

So you can get smart --- answers from QRZed members.

N1RKW
02-16-2009, 06:02 PM
At 144.000 mhz, 1/4 wave is 19.500 inches. At 146.000 it's 19.232 inches. At 148.000 you're looking at 18.972 inches for a 1/4 wave. In case you want to know, 145.000 = 19.365" and 147 = 19.102".
You won't get much more precise than that.
Your swr, however, isn't entirely controlled by your antenna element length. For example, I've got a 2 meter antenna that has low swr on 440 mhz and radiates like a champ, even though the driven element is more than a wavelength long.

KD8DEY
02-16-2009, 06:02 PM
468/f is for hf

475/f is used for vhf

so at 144mhz it would be 19.79 inches to be resonant
at 148 mhz it would be 19.25 "

that is why most "Books" list 19.5 inches for a 1/4 wave antenna for 2m (center of the band)
:D

KL7AJ
02-16-2009, 06:05 PM
At 144.000 mhz, 1/4 wave is 19.500 inches. At 146.000 it's 19.232 inches. At 148.000 you're looking at 18.972 inches for a 1/4 wave. In case you want to know, 145.000 = 19.365" and 147 = 19.102".
You won't get much more precise than that.
Your swr, however, isn't entirely controlled by your antenna element length. For example, I've got a 2 meter antenna that has low swr on 440 mhz and radiates like a champ, even though the driven element is more than a wavelength long.

True...and if it's over a perfect ground plane, the impedance will be 36 ohms, not 50!

eric

WB2WIK
02-16-2009, 06:10 PM
>G4ILO;1488631]I always thought it was 19 inches. That's what the formulas say.<

::That's what it is.

>But when I put one of those 19 inch telescopic whips on my FT-817 I got three bars of SWR. By clipping some wire to the end I got it down to no bars with a total length of just over 22 inches. And not only that, the strength of the repeater I was monitoring came up.<

::Something in the system (could be inside the FT-817) is introducing reactance, so you need a reactive antenna of opposite polarity to cancel that and provide a better match. Simple as that.

>I have checked this out with an external SWR meter and an antenna analyzer, and 22 - 23 inches definitely seems to give a better match.<

::You cannot properly analyze this, because that involves removing the whip from the FT-817 and installing it on something else. As soon as you do this, the entire system is changed. You cannot replicate an FT-817 by mounting the whip on an SWR bridge or antenna analyzer and connecting it with a cable -- it's a completely different situation. Motorola always tuned their portable (HT mounted) antennas to provide a proper match when mounted on the HT. None of them matched worth a darn if mounted someplace else.

>So why do they make these telescopic whips 19 inches max?<

::19" is a 1/4-wave at 146 MHz and it's the correct length. Forcing the whip length to something else is only to accommodate reactance in the system which shouldn't be there.

WB2WIK/6

PA5COR
02-16-2009, 06:11 PM
Mounted on a flat surface, yes ---
Mounted on a sloping surface no :D ^



True...and if it's over a perfect ground plane, the impedance will be 36 ohms, not 50!

eric

WB2WIK
02-16-2009, 06:12 PM
Mounted on a flat surface, yes ---
Mounted on a sloping surface no :D ^

::And, mounted on an FT-817, heck no.

The FT-817, like all other small "hand held" type transceivers, isn't any sort of tuned ground plane at 146 MHz or any other frequency.

KW7DSP
02-16-2009, 06:19 PM
1/2 meter (2 meters / 4)



I couldn't resist.

KL7AJ
02-16-2009, 06:26 PM
Mounted on a flat surface, yes ---
Mounted on a sloping surface no :D ^

Depends on whether the whip slopes with the surface. :)

WA9SVD
02-16-2009, 07:13 PM
1/2 meter (2 meters / 4)



I couldn't resist.

Half a meter???:confused: Is that digital or analogue?:p

K9STH
02-16-2009, 09:06 PM
Also, a telescoping whip has sections of varying diameter and that also affects the length of the antenna. This "taper" effect will normally make the antenna longer than when it is made from material of a constant diameter.

There is a program that is part of the YagiMax "suite" that calculates the changes in length due to varying diameter material.

Glen, K9STH

WA4TM
02-16-2009, 09:53 PM
So,, if the extra wire works for you, use it, or not.

Got duct tape? :D

KM3F
02-16-2009, 10:22 PM
The 1/4 wave length at any operating frequency is based on an antenna configuration for an antenna not on the basis of connecting a radiating element to a radio's connector.
For instance, on a hand held that has no chance of presenting a proper other "half" to the radiating element, the holder's body is the other half and the radiating element is inductance to make it physically short.
The result of this configuration satisfies within variation, the 50 ohm match the transmitter requires but at a great LOSS in efficiency.
Bottom line is a best or close to perfect match never is an indication of the best performance that is possable with a proper antenna.
On your setup, you may see the SWR change just by putting your hand on the cabinet or from some other action showing it is not a proper set of conditions for antenna match and operation.
There are no short cuts that work proper most of the time.

AD5MB
02-16-2009, 10:43 PM
The result of this configuration satisfies within variation, the 50 ohm match the transmitter requires

or

the OPs radio has an output impedance that is not 50 ohms.

or

his SWR meter is less than accurate.

If it works, go with it.

G0GQK
02-16-2009, 10:49 PM
Seems to me they count numbers different in the US as well ! Isn't 19.79 inches quite a bit shorter than 22 inches, whichever formula you are using !

There seems to be in the radio amateur world various myths, which set in stone, are passed from one generation to another. For instance, there are more precise formula's than 468/ F. Mhz, three of them, for good, bad and average ground, but nobody uses them.

It's also said that a quarter wave radial has to be at a 45 degree angle for a 50 ohm feed, not true, unless its fed through an ATU the nearest on HF anyway is 1:3 SWR. I've had SWR 1:3 on horizontal quarter wave counterpoise radials five feet above ground.

Antenna's have to be tuned to where they are located, and like violins in changeable weather, they go out of tune.

G0GQK

ZL3GSL
02-17-2009, 12:50 AM
At about 19", the antenna will be resonant. That doesn't mean that the SWR will be 1:1 when it's connected to a set with a nominal 50 ohm output. It's only half of a dipole, so is more like 20 ohms than 50.

"Three bars" is probably an SWR of about 2:1 , which is probably correct. Your extra three inches is adding some ohms of (inductive) reactance to increase the impedance a bit to lower your SWR.

VK6ZGO
02-17-2009, 01:01 AM
1/2 meter (2 meters / 4)



I couldn't resist.

You got in before me:D

Actually 1/4 wave is 520.83 mm at 144MHz, & 506.76 mm at 148 MHz (free space)

But seriously, how is it that people don't see the relationship between the band "2 meters" & the length "2 metres"?

It reminds me of a friend of mine who when asked if he had "2 meters" in his car, said "Yes, a Speedometer & an Ammeter!"

73 VK6ZGO

G4ILO
02-17-2009, 09:57 AM
You got in before me:D

Actually 1/4 wave is 520.83 mm at 144MHz, & 506.76 mm at 148 MHz (free space)

But seriously, how is it that people don't see the relationship between the band "2 meters" & the length "2 metres"?

It reminds me of a friend of mine who when asked if he had "2 meters" in his car, said "Yes, a Speedometer & an Ammeter!"

73 VK6ZGO

When choosing the title of this thread I forgot how narrow minded and literal some amateurs are. It was simply meant to summarize the question of whether a quarter wave whip should be 19 inches, or 22 inches, or something else. Still at least I gave some people the opportunity to crow about how superior they are because they thought they knew something I didn't.

Thanks to all those replied with sensible answers.

It still begs the question of why do they make those little SMA or BNC telescopic whips for 2m 19 inches long instead of 22 inches or so, when it would seem that the extra length would allow them to match the rigs they are designed to be used with so much better?

WB5ITT
02-17-2009, 05:02 PM
I always thought it was 19 inches. That's what the formulas say. But when I put one of those 19 inch telescopic whips on my FT-817 I got three bars of SWR. By clipping some wire to the end I got it down to no bars with a total length of just over 22 inches. And not only that, the strength of the repeater I was monitoring came up.

I have checked this out with an external SWR meter and an antenna analyzer, and 22 - 23 inches definitely seems to give a better match. So why do they make these telescopic whips 19 inches max?


This may have been answered correctly before but yes a 1/4wave for 2m is 19inches....now when trying to make a 1/4 ertical, the formula is 234/f(mhz) = length in feet OVER PERFECT GROUND PLANE. The 817 is NOT...thus the length of the vertical had to be changed to compensate for the poor ground plane, etc......IF You have put a 19in wire from antenna GROUND and let it hang, then the vertical would have been 19in.....but the case of the radio sucks for a ground plane (except maybe UHF).....

Chris
WB5ITT

KA9VQF
02-17-2009, 05:34 PM
Its kind of the same as if you were trying to decide which weapon to use in your next assignation attempt.

You could use a .50 cal snipers rifle and shoot your victim from a mile or maybe more away or you could use a stubby barrel .22 at closer range.

A sawn off double barrel .12 gauge might be best if you decide to go close. That way you could miss a little and still get ‘em. {better band width}

The 19 and a half inches puts you pretty much in the middle of the 2M FM part of the band so you are fairly close no matter what frequency in that part of the band you are transmitting on.

When I build a wave ground plain antenna for 2M I usually put a little loop in the top part of the vertical and make it 19&” to where it goes into the SO-239. I generally make the radial wires 22” to the loop in the end of them.

This makes a good enough match on enough of the band that you can use my antennas without the worry that you will have enough of an SWR mismatch that the rig will run hot even on long QSO’s. You really don’t have to worry much that the finals will be oozing out the back of the rig anytime soon.

Since you are using a handheld rig, I wonder if you were to take an iron supplement for a few weeks if your SWR would change.

VK6ZGO
02-19-2009, 08:04 AM
When choosing the title of this thread I forgot how narrow minded and literal some amateurs are. It was simply meant to summarize the question of whether a quarter wave whip should be 19 inches, or 22 inches, or something else. Still at least I gave some people the opportunity to crow about how superior they are because they thought they knew something I didn't.

Thanks to all those replied with sensible answers.

It still begs the question of why do they make those little SMA or BNC telescopic whips for 2m 19 inches long instead of 22 inches or so, when it would seem that the extra length would allow them to match the rigs they are designed to be used with so much better?

Sorry to come across as "crowing" .Being literal minded is not a bad thing if it

leads you to working out things from first principles.

1/4 wave at 144Mhz (520.83mm) works out to be approx 20.5"

& 1/4 wave at 148Mhz (506.76mm) is approx 19.95"

Neither of these are close to 22"(& they are still free space dimensions)

Allowing for a reduction in length for physical antennas as compared with

free space, they become:-

For 144MHz---19.47"

& for 148 MHz ---18.95"

Awfully close to 19".

But you knew all this!

The posters that say it is because the FT817 is a lousy groundplane got it

right,as you have no doubt recognised.

Have you tried the antenna at both extremes of your 2m band to see if it

gets better at the top end?

I wonder if it is more or less optimised for the US (& Australian) markets

where we have the full 4 MHz.

It's a dumb idea if it is!

73 VK6ZGO

N8CPA
02-19-2009, 02:26 PM
Only if measured in footlongs per fortnight.:)

"Five.
Five dollar.
Five dollar footlong..."

WA9SVD
02-19-2009, 06:26 PM
Only if measured in footlongs per fortnight.:)

I thought that was supposed to be FURLONGS per Fortnight. And my car holds approximately one Firkin of gasoline... ;)


the speed of light in vacuum is approximately 1.80310^12 furlongs per fortnight, or rather 1.803 terafurlongs per fortnight; YMMV

K7MH
02-19-2009, 07:13 PM
[QUOTE=WA9SVD;1491842]I thought that was supposed to be FURLONGS per Fortnight. And my car holds approximately one Firkin of gasoline... ;)

I have a Chevy Silverado truck, and it holds one Firkin LOT of gasoline!!

K7UF
02-19-2009, 08:02 PM
When choosing the title of this thread I forgot how narrow minded and literal some amateurs are. It was simply meant to summarize the question of whether a quarter wave whip should be 19 inches, or 22 inches, or something else. Still at least I gave some people the opportunity to crow about how superior they are because they thought they knew something I didn't.

Thanks to all those replied with sensible answers.

It still begs the question of why do they make those little SMA or BNC telescopic whips for 2m 19 inches long instead of 22 inches or so, when it would seem that the extra length would allow them to match the rigs they are designed to be used with so much better?

Its not just amateurs.

Lets assume you want a 1/4 wave at 145 MHz (middle of the UK 2M band).
Lets also work in meters rather than inches.

Velocity of light is 300,000,000 m/s (well, actually its 299,792,458 m/s, but we won't worry too much about that.

So, the wavelength of your 145Mhz signal is 300/145 = 2.069m

A quarter of that is 0.517m (517mm, 20.534 inches).

That is based upon the velocity of light in a vacuum.
Since we don't live in a vacuum, and there are other factor affecting the velocity of propagation of RF along a piece of wire, it will actually travel somewhat slower, so the actual wavelength will be shorter.

A rough estimate of the speed of light along your piece of copper in the damp and dismal air of England might be about 95% of its speed in a vacuum.

So the actual wavelength end up being 419mm (19.475").

So far, so good. We know where the ~19" comes from.

Now, for a 1/4 wave vertical (whip) to work correctly as an antenna, it needs to be sitting on a perfect (or as close as we can get) groundplane.

The roof of your car is a somewhat reasonable groundplane at these frequencies. So a 1/4 whip on your car roof would be resonant at 145MHz if it were ~ 19.5" long.

A perfect 1/4 vertical antenna on a perfect groundplane has an impedance of ~ 30 ohms (don't remember the actual value off the top of my head, but 30-ish is about right). Fortunately, a typical car roof is not a perfect groundplane, so the actual impedance is a bit higher. The 50 ohm impedance of your transceiver will be about right.

Now, lets take that whip and drop it onto your hand-held. What happened to the groundplane? It essentially went away. The HT and your hand holding it form a somewhat lousy groundplane. The whip is still 1/4 wave, but the input impedance will have gone all over the place. It will be far from 50 ohms resistive. It will probably be much higher, and have some serious reactive components.

What you would normally do to match an antenna is to add reactive components. What you are doing by lengthening the antenna is exactly that. The antenna is no longer resonant, but its a better match.

As to why do vendors sell 19" whips? Would you believe that its because people buy them?

In reality, there is no "right" length. On a different HT you might find that you have to reduce the length of the whip to get a good match.

WB3BEL
02-19-2009, 08:54 PM
The required length of a radiating element to appear to be a quarter wavelength long at any frequency depends on its diameter.

The 19.5 inch length quoted for 146 MHz is only true for small diameter radiators.

Here is a plot of how the length gets shorter if you use larger diameter radiating elements.

It may not be so critical if you are just building some simple vertical, but if you are making a phased array or yagi, and you get it wrong it can be a disaster.

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