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Horizontal loop antenna



 
 
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  #11  
Old February 19th 08, 09:26 PM posted to rec.radio.amateur.antenna
Roy Lewallen
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Posts: 1,374
Default Horizontal loop antenna

I'm missing something here. I don't see anywhere in the response which
explains how "radiation is a function of loop area" and why increasing
the loop circumference would be advantageous.

Richard Harrison wrote:
Roy Lewallen, W7EL wrote:
"Can you please explain that a little more? (Radiation is a function of
loop area.)"

Not being a typist, I`ll refer you to "TV And Other Receiving Antennas"
by Arnold Bailey. On pages 407 and 408 Bailey gives two formulas for
computing the antenna resistance for a loop antenna.

On page 408, Bailey has Fig. 8-14 which plots radiation resistance (the
stuff we build antennas for) versus the loop perimeter in wavelengths.
For a square closed loop of one wavelength perimeter, the graph
indicates about 50 ohms.


Perhaps you build antennas for radiation resistance, but I think most
people build them to maximize radiation in some direction. In any case,
please exclude me from the "we" in your statement about what "we" build
antennas for. You said "radiation is a function of loop area" and
recommended a larger circumference. Equations are useless unless you
understand how to apply them. So can you use those equations to show us
how much more a loop of 800 foot circumference will radiate than one
with a 400 foot circumference? You can assume 100 watts applied to each,
and either zero wire resistance or any reasonable value.

Bill Orr, W6SAI in "All About Cubical Quad Antennas" gives the full-wave
vertical loop antenna an impedance of 125 ohms on page 15.

On page 14, Orr writes:
"For purposes of illustration, the two wire folded dipole may be "pulled
open" to a diamond-shaped loop fed at the bottom point. If this
distortion of the loop is continued the antenna will become a shorted
transmission line."

A perfect circle is the geometric shape enclosing the most area for a
given perimeter. The more corners a closed figure has, the more closely
it usually approximates a circle. That is why I commented on an octagon
versus a square.


Which brings me back to the question I asked,

Is the radiation pattern the same for a long skinny loop as for a round
one, as long as the enclosed area is the same?


Roy Lewallen, W7EL
  #12  
Old February 19th 08, 09:39 PM posted to rec.radio.amateur.antenna
Cecil Moore[_2_]
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Posts: 3,522
Default Horizontal loop antenna

Roy Lewallen wrote:
I'm missing something here. I don't see anywhere in the response which
explains how "radiation is a function of loop area" and why increasing
the loop circumference would be advantageous.


What you seem to be missing is that Richard H. said
absolutely nothing about radiated *power*. The radiation
*pattern* is certainly a function of loop area.
--
73, Cecil http://www.w5dxp.com
  #13  
Old February 19th 08, 11:44 PM posted to rec.radio.amateur.antenna
Sum Ting Wong
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Posts: 34
Default Horizontal loop antenna

On Mon, 18 Feb 2008 19:31:00 -0800, Roy Lewallen
wrote:

Putting the balun the input has the disadvantage of
making the tuner chassis hot.


Roy,

Thanks for ANOTHER lesson. I have a SGC-230 tuner in the RV that I
use to tune a wire when we go camping. I think the metal mounting
brackets are isolated from the floating tuner chassis and that you
have to use a grounding lug on the bottom. Never verified that since
I have always used the ground lug anyway. I was thinking of adding
ferrites to the input side of the tuner and then using it to tune a
dipole fed with 450 ohm window line. Maybe I'll just stick with the
longwire.

I noticed the guy who wrote that article mounted his tuner on an el
cheapo cutting board, probably just for that reason.

S.T.W.
  #14  
Old February 20th 08, 12:18 AM posted to rec.radio.amateur.antenna
Cecil Moore[_2_]
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Posts: 3,522
Default Horizontal loop antenna

Sum Ting Wong wrote:
I have a SGC-230 tuner in the RV ... I was thinking of adding
ferrites to the input side of the tuner and then using it to tune a
dipole fed with 450 ohm window line.


My SG-230 tuner manual says that is a no-no.
It says: "The coupler must be located *at the antenna*.
Never use a feed line or coaxial cable at the output
of the antenna coupler."
--
73, Cecil http://www.w5dxp.com
  #15  
Old February 20th 08, 02:45 AM posted to rec.radio.amateur.antenna
Sum Ting Wong
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Posts: 34
Default Horizontal loop antenna

On Tue, 19 Feb 2008 23:18:14 GMT, Cecil Moore
wrote:

Sum Ting Wong wrote:
I have a SGC-230 tuner in the RV ... I was thinking of adding
ferrites to the input side of the tuner and then using it to tune a
dipole fed with 450 ohm window line.


My SG-230 tuner manual says that is a no-no.
It says: "The coupler must be located *at the antenna*.
Never use a feed line or coaxial cable at the output
of the antenna coupler."


Cecil,

Saw that and wondered about isolating the coax and DC power with the
ferrite and maybe changing the rules about doing that. Since you're a
whole lot sharper than I am on this stuff, what do you figure the
reason is for not tuning a balanced feeder with it? That would make
things sooooooooo much easier if it worked.

Thanks.

S.T.W.
  #16  
Old February 20th 08, 03:21 AM posted to rec.radio.amateur.antenna
Dave Platt
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Posts: 464
Default Horizontal loop antenna

I have a SGC-230 tuner in the RV ... I was thinking of adding
ferrites to the input side of the tuner and then using it to tune a
dipole fed with 450 ohm window line.


My SG-230 tuner manual says that is a no-no.
It says: "The coupler must be located *at the antenna*.
Never use a feed line or coaxial cable at the output
of the antenna coupler."


On the other hand, page 16 of the SGC-230 manual PDF specifically
shows a balanced dipole, being fed through "balanced line feeders,
300-600 ohms, up to 40 feet".

The ladder line is shown as being connected to the antenna "hot"
terminal, and to the case of the coupler.

The manual is quite clear about not using a coax feedline above the
antenna, or having a feedline out to a longwire antenna which has more
than a very small amount of capacitance to ground. The information
shown on page 16 suggests that you may be able to bend this
restriction if you're using a balanced feedline, although I suspect
that you'll still want to make sure that you've got low capacitance
between ground and the hot side of the line (keep the feedline well
away from metal, and use high-quality feedthrough insulators).

I don't know about using a balun at the base of the feedline... this
will (almost of necessity) require the use of a length of
low-impedance feedline of some sort, and this could be exactly the
sort of thing that prevents the SGC from matching the line properly.
Using a choke on the coax-and-control side might work better, although
this too seems to be something that SGC discourages.

I haven't been terribly happy with the SGC tuner I picked up
(admittedly for cheap, at a ham swap-meet). It's an early-model SGC
230, and its tuning algorithm seems very finicky. In particular, it
won't tune into any but the easiest loads when fed by my Kenwood
TS-2000. I suspect that the reason is that the TS-2000's high-SWR
foldback circuit is rather aggressive, and chops the power down to well
under 10 watts anytime the rig sees an SWR of more than 2:1 or so.
The fluctuating transmitter power seems to confuse the coupler's
matching algorithm... no big surprise there. Seems to work better
when fed from a simple TenTec Scout, which has a different high-SWR-
protection mechanism that doesn't cause the power to flop around as
much.

Anyhow... my impression is that the SGC tuners are best suited for
their original application - feeding a whip or longwire, whilst being
securely bolted and multiply-grounded to a Big Metal Vehicle Chassis.
They also seem to do OK feeding verticals over a big bed of radials.
I just don't think they're all that well suited to feeding balanced
antennas.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
  #18  
Old February 20th 08, 04:31 AM posted to rec.radio.amateur.antenna
Roy Lewallen
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Posts: 1,374
Default Horizontal loop antenna

Sum Ting Wong wrote:
On Mon, 18 Feb 2008 19:31:00 -0800, Roy Lewallen
wrote:

Putting the balun the input has the disadvantage of
making the tuner chassis hot.


Roy,

Thanks for ANOTHER lesson. I have a SGC-230 tuner in the RV that I
use to tune a wire when we go camping. I think the metal mounting
brackets are isolated from the floating tuner chassis and that you
have to use a grounding lug on the bottom. Never verified that since
I have always used the ground lug anyway. I was thinking of adding
ferrites to the input side of the tuner and then using it to tune a
dipole fed with 450 ohm window line. Maybe I'll just stick with the
longwire.

I noticed the guy who wrote that article mounted his tuner on an el
cheapo cutting board, probably just for that reason.

S.T.W.


And you can also thank Tom Rauch, W8JI who questioned my incorrect
belief, causing me to re-think it.

Roy Lewallen, W7EL
  #19  
Old February 20th 08, 04:55 AM posted to rec.radio.amateur.antenna
Buck[_2_]
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Posts: 118
Default Horizontal loop antenna - Dual Wire

I seem to remember an article about someone taking a full-wave
horizontal loop antenna and making two loops (or was it a half-wave
loop?). The second loop caused the antenna to match 50 ohms so it
could be fed with coax and (I think) it operated multiple bands.
Anyone remember this article and know where it might be?

Thanks
Buck
N4PGW
--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."
  #20  
Old February 20th 08, 05:18 AM posted to rec.radio.amateur.antenna
Cecil Moore[_2_]
external usenet poster
 
Posts: 3,522
Default Horizontal loop antenna

Sum Ting Wong wrote:
Saw that and wondered about isolating the coax and DC power with the
ferrite and maybe changing the rules about doing that. Since you're a
whole lot sharper than I am on this stuff, what do you figure the
reason is for not tuning a balanced feeder with it?


I was so sharp, I called SGC. :-) They said if the
transmission line accidentally opened up, the SG-230
could develop 30,000 volts at its output. If it was
mounted in an attic, for instance, it could burn the
house down. I think they are worried about being sued.
Moral: If you decide to use transmission line on the
output, put it in a location that will not matter if
it catches on fire.
--
73, Cecil http://www.w5dxp.com
 




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