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Author Topic: Any significance of slighly different ground radial lengths?  (Read 1535 times)
K7RNO
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Posts: 279




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« on: April 02, 2013, 02:42:55 PM »

I got this older 1/4w 2m ground plane antenna, and it works well with my HT. It has three ground radials, which are all slightly different in effective length:
18 1/8"
18 1/2"
18 5/8"

Is there any significance in this?
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73,
aRNO
NAQCC #6870, SKCC #11131
W5CPT
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Posts: 557




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« Reply #1 on: April 02, 2013, 03:48:39 PM »

Not that you will ever notice.

Clint - W5CPT -
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KB4QAA
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Posts: 2367




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« Reply #2 on: April 02, 2013, 05:07:55 PM »

Nope. 
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WB6BYU
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Posts: 13234




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« Reply #3 on: April 02, 2013, 05:21:12 PM »

No.
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AA4HA
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Posts: 1424




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« Reply #4 on: April 02, 2013, 06:52:29 PM »

A question on the radial lengths that were provided (right around 18.5"). That puts the quarter wave resonance right around 159 MHz, does the attachment point and any other body length for the base of the antenna add to the quarter wave length?

For 146 MHz the quarter wave length should be 20.2". Are the additional 2.7" coming from those additional mechanical components between the base of the driven element and where the 1/4 wave radials are threading into the antenna base?

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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
K7RNO
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Posts: 279




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« Reply #5 on: April 02, 2013, 10:28:23 PM »

A question on the radial lengths that were provided (right around 18.5"). That puts the quarter wave resonance right around 159 MHz, does the attachment point and any other body length for the base of the antenna add to the quarter wave length?

For 146 MHz the quarter wave length should be 20.2". Are the additional 2.7" coming from those additional mechanical components between the base of the driven element and where the 1/4 wave radials are threading into the antenna base?



Good questions. This antenna's base is like a thick espresso cup upside-down. The radials angle into its flank, so there is more material before their length meets the shield of the coax, if that is how you would measure. Not 1.7" more, though.

Also, the driven element rises only 19.5" above the bottom of the cup. However, since this looks like a manufactured, not like a home brew, antenna, I assume that somebody tuned it right. And it worked quite effectively together with my HT.

So this will be my bring-along outdoors antenna for when I hike (or ride/drive) up the mountains.
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73,
aRNO
NAQCC #6870, SKCC #11131
GW3OQK
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Posts: 145




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« Reply #6 on: April 03, 2013, 01:20:19 AM »

I have seen a marine vhf band ant made like that. Centre freq about 159 MHz. The elements screwed into the centre piece and made of stainless steel.
Andrew
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AA4HA
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Posts: 1424




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« Reply #7 on: April 03, 2013, 06:15:16 AM »

I too would assume that a commercial antenna manufacturer would know the formula for quarter wave length and get it right on a product. I just took a look at the Yaesu ATAS-120 with the counterpoise kit that I use for mobile/ portable operation (on a camera tripod). The 2 meter (counterpoise/ ground radial) is 23" long.

At 23" the resonant frequency is 128 MHz, not 146 MHz where the center of the 2 meter band is. "What is good for the goose should be good for the gander" as far as element lengths are concerned. If we were cutting the element length on a quarter wave whip antenna that much of a difference would be significant. Why does it not appear to be so with their choice of a radial (counterpoise) element?

For your antenna it is shorter than expected, for the antenna I have it is longer than expected. Unless I have been taking crazy-pills (again) or the laws of the universe are subject to change the resonant length should be predictable and follow the quarter wavelength rule.

I have an interest as I have been looking at modifying the counterpoise ring of the Yaesu "ATBK-100" kit to take additional counterpoise elements. I want to go with (3) 70cm radials, (3) 2m radials, (3) 6m radials and (3) wire radials using ribbon cable that are cut at quarter wave lengths for 10,12,15,17,20,30,40m. The wire radials are also going to act as guy lines for my tripod mount when I am at base-camp and are each 10m (33 ft) long.

The standard ATBK-100 counterpoise kit has (1) 70cm radial, (1) 2m radial and (2) 6m (coil loaded) radials.
« Last Edit: April 03, 2013, 07:05:04 AM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WB6BYU
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Posts: 13234




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« Reply #8 on: April 03, 2013, 07:16:42 AM »

Radials don't actually have to be exactly 1/4 wavelength long to work, though
that is convenient.  If you imagine the ground plane as a dipole with the bottom
portion flared out, we know that a dipole can be fed off-center somewhat and
still work.

For example, I have a design published by Woody Smith W6BCX (inventor of
the Bobtail Curtain et. al.) that uses a 18" whip and 24" radials for 2m, and
it works great.  The original intent was to reduce common mode currents,
though I haven't verified whether it actually does that or not.  (I happen to
like that version because I can cut a single piece of brazing rod in half for two
vertical elements.)  I've also seen 40m verticals using a longer radiator and two
shorter radials to get a better match to 50 ohms.  With a roof-mounted 14 AVQ
we used to adjust the radial lengths to move the SWR curve because it was
easier than getting up to retune the antenna.

So what is important is that the radiator + radials be resonant, even if they
aren't exactly the same length.  With the shorter radiator / longer radials
version the total length is longer than the standard version, and the resonant
radial length is different for 2 radials than for 4 radials, but the usual pragmatic
tuning approach is sufficient to get a low SWR.

Of course, common mode currents need to be considered even when all the
elements are exactly a quarter wave, and can more of an issue as the feedpoint
is moved off center.
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RFRY
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Posts: 311


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« Reply #9 on: April 04, 2013, 03:28:38 PM »

... If you imagine the ground plane as a dipole with the bottom portion flared out, we know that a dipole can be fed off-center somewhat and still work. ...

The difference being that a vertical monopole driven against symmetrically-arrayed horizontal wires either buried in the earth, or elevated above the earth and used as a counterpoise in a "ground plane" antenna produce no useful far-field radiation, themselves.  All of the useful radiation is produced by the vertical conductor of such antennas.

OTOH, both sections of a linear dipole produce useful far-field radiation, whether or not that dipole is center fed.

R. Fry
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