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Has anybody had any experience using a helical antenna for
transmissions. I put a small helical antenna (65' 14# wire, 172 turns
on 1-3/8" dowel) on my antenna tuner: on 40 meters (7MHz) the SWR was
about 8:1; on 15 meters (21MHz) the SWR was 1.6:1). I thought the 40M
would tune better than the 15M because 65' is about 40M's 1/2 wl.

My antenna tuner manual suggested a balun to tune in any band that
doesn't initially tune-up. I'm thinking the ubiquitous 4:1 balun.

Suggestions, comments?

Tnx,
--
MGFoster:::mgf00 at earthlink decimal-point net (KI6OFN)
Oakland, CA (USA)
** Respond only to this newsgroup. I DO NOT respond to emails **

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MGFoster wrote:
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Has anybody had any experience using a helical antenna for
transmissions. I put a small helical antenna (65' 14# wire, 172 turns
on 1-3/8" dowel) on my antenna tuner: on 40 meters (7MHz) the SWR was
about 8:1; on 15 meters (21MHz) the SWR was 1.6:1). I thought the 40M
would tune better than the 15M because 65' is about 40M's 1/2 wl.

My antenna tuner manual suggested a balun to tune in any band that
doesn't initially tune-up. I'm thinking the ubiquitous 4:1 balun.

Suggestions, comments?

Tnx,

Only a couple of comments.

1. Winding a half wavelength of wire doesn't make an antenna which acts
like a half wavelength antenna. Turn-to-turn coupling makes the total
wire length a lot less important than other factors like turn diameter
and spacing.
2. My experience with using helical antennas is very limited, so other
folks can much better tell you what combinations might provide you with
a good match. But a helical antenna that's physically quite a bit
shorter than a half wavelength will have a low radiation resistance. So
if you get a good match without a tuner, it means that there's enough
loss to raise the feedpoint resistance to 50 ohms. Another good
indication of loss is the bandwidth. If it has low loss, the bandwidth
will be very narrow; a wide bandwidth means more loss. Most people,
though, run at least several times the power needed to communicate, so
trading some efficiency for bandwidth, easy matching, and compact size
might be a worthwhile exchange in a lot of cases.

If the loss is low, the feedpoint resistance at resonance would be
considerably less than 50 ohms, so a 4:1 balun, with the high impedance
side toward the tuner, might make things easier for the tuner.

Roy Lewallen, W7EL

Roy Lewallen wrote:
1. Winding a half wavelength of wire doesn't make an antenna which acts
like a half wavelength antenna.

A helical antenna section can be treated as a piece
of transmission line with a velocity factor and a
characteristic impedance. Knowing the velocity factor
allows one to calculate the approximate length of
the section. On my web page, I have an EXCEL file
that will estimate the VF and Z0 of a helical
antenna: http://www.w5dxp.com/coilZ0VF.xls

To see how this might work, let's assume a 1/2WL
helical dipole for 40m using a helical of 2 inches
diameter and 2 turns per inch on 7.2 MHz. If the
value at A7 on the spreadsheet is less than one,
the VF and Z0 will be in the correct ballpark
- probably within 15%.

For such a coil, the VF is calculated to be 0.207.
1/2WL at 7.2 MHz is 68.3 feet. So how long should
the coil be to be 1/2WL long?

68.3 feet times 0.207 = ~14 feet

Of course, this is not an exact length and must
have turns subtracted (or added) to bring it to
resonance on 7.2 MHz. Since end effects are not
included, the dipole will probably resonate on
a frequency lower than 7.2 MHz. Removing turns
will then bring it to resonance.

Use half of the above for a 1/4WL vertical
section approximately 7 feet high.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Roy Lewallen wrote:
1. Winding a half wavelength of wire doesn't make an antenna which
acts like a half wavelength antenna.

A helical antenna section can be treated as a piece
of transmission line with a velocity factor and a
characteristic impedance. Knowing the velocity factor
allows one to calculate the approximate length of
the section. On my web page, I have an EXCEL file
that will estimate the VF and Z0 of a helical
antenna: http://www.w5dxp.com/coilZ0VF.xls

To see how this might work, let's assume a 1/2WL
helical dipole for 40m using a helical of 2 inches
diameter and 2 turns per inch on 7.2 MHz. If the
value at A7 on the spreadsheet is less than one,
the VF and Z0 will be in the correct ballpark
- probably within 15%.

For such a coil, the VF is calculated to be 0.207.
1/2WL at 7.2 MHz is 68.3 feet. So how long should
the coil be to be 1/2WL long?

68.3 feet times 0.207 = ~14 feet

Of course, this is not an exact length and must
have turns subtracted (or added) to bring it to
resonance on 7.2 MHz. Since end effects are not
included, the dipole will probably resonate on
a frequency lower than 7.2 MHz. Removing turns
will then bring it to resonance.

Use half of the above for a 1/4WL vertical
section approximately 7 feet high.


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Thanks Roy and Cecil. I'll work on this using your
comments/suggestions.

I'm in a vy antenna unfriendly area: an apartment in the air shaft of
the bldg. So I'm abt 40' from free-air on all four sides. Fortunately,
I'm on the top floor, so I'm trying to get a "stealth" antenna to place
on the roof: something that would be easy to set up/take down. The
helical fit that description. But, I'm running QRP (abt 5W) and, taking
into consideration what Roy said abt power & efficiency, I'm probably
not getting anything out.

One good thing abt this antenna is, for it's size, it does receive
better than the random-length wire (abt a 50' rectangle) I had been
using inside my apartment. It does have good bandwith - but, as Roy
stated this indicates low radiation resistance.

Cecil, I'll work w/ your s/s and play around some more w/ the helical
(just for the fun of it, ya know ;-) ).

If this doesn't work out I'll try a linearly loaded dipole that N5ESE
has on his website.

Thanks to both for your advice.

73

--
MGFoster:::mgf00 at earthlink decimal-point net (KI6OFN)
Oakland, CA (USA)


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MGFoster wrote:
Cecil, I'll work w/ your s/s and play around some more w/ the helical
(just for the fun of it, ya know ;-) ).

You will radiate more power if you make the center half
of the antenna straight wire and use the ends of the
antenna for the helical part. If you haven't considered
top hats on the antenna ends, please do so. The following
would be a pretty good antenna, in fixed font with top
hat loading at the ends.

| coil coil |
+---////////--------FP--------////////---+
| |

--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
MGFoster wrote:
Cecil, I'll work w/ your s/s and play around some more w/ the helical
(just for the fun of it, ya know ;-) ).

You will radiate more power if you make the center half
of the antenna straight wire and use the ends of the
antenna for the helical part. If you haven't considered
top hats on the antenna ends, please do so. The following
would be a pretty good antenna, in fixed font with top
hat loading at the ends.

| coil coil |
+---////////--------FP--------////////---+
| |


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Isn't that a linearly loaded dipole?

I saw a picture of a similar antenna in some book I was browsing - the
illustration didn't show a center wire, just the coils and the top-hats.

Would you keep the wooden dowel form (for the coils), or would you let
them be air-coils (how to keep them from separating [tape])? What
lengths would you use for the wire/coils? What type feed line would you
use: twin lead or coax? Am I asking too many Qs?

Wait a minute... I've seen examples of this type of ant. on other
websites. I'll look those up and you won't have to answer these Qs,
which have probably been asked many times before.

N5ESE shows a linearly load dipole that is about 12 feet long (6 ft per
pole), twin lead fed with 14 inches of zig-zag wire at the ends. He
reports that he had pretty good results w/ this ant. on 15M and 20M (the
ant. was inside his bedroom at the time of the test).

http://www.io.com/~n5fc/notebk_ant.htm

Thanks,
--
MGFoster:::mgf00 at earthlink decimal-point net (KI6OFN)
Oakland, CA (USA)
** Respond only to this newsgroup. I DO NOT respond to emails **

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MGFoster wrote:
Cecil Moore wrote:
| coil coil |
+---////////--------FP--------////////---+
| |

Isn't that a linearly loaded dipole?

No, it is a loaded dipole. It uses inductive loading
and top hat capacitive loading. "Linear", in this
context, means "evenly distributed" loading.

Would you keep the wooden dowel form (for the coils), or would you let
them be air-coils (how to keep them from separating [tape])? What
lengths would you use for the wire/coils? What type feed line would you
use: twin lead or coax?

I would use PVC pipe and run a Dacron "messenger line"
through the center for mechanical support.

For the coils, you will need approximately double the
length of the straight wire. Design the coils so you
can add or subtract turns. Or design the top hats to
be adjustable. Or adjust the length of the straight
wires.

The feedpoint impedance will probably be lower than
50 ohms. For single-band operation, coax will probably
work well with an antenna tuner.
--
73, Cecil http://www.w5dxp.com

On Thu, 07 Feb 2008 13:13:26 -0800, MGFoster wrote:

I'm in a vy antenna unfriendly area: an apartment in the air shaft of
the bldg. So I'm abt 40' from free-air on all four sides. Fortunately,
I'm on the top floor, so I'm trying to get a "stealth" antenna to place
on the roof: something that would be easy to set up/take down. The
helical fit that description. But, I'm running QRP (abt 5W) and, taking
into consideration what Roy said abt power & efficiency, I'm probably
not getting anything out.

You are getting out, however, by your low enthusiasm, it sounds like
you don't get much back in response for tossing out CQs. This may be
simply a matter of persistence because what you say more below
suggests not all is for naught:

One good thing abt this antenna is, for it's size, it does receive
better than the random-length wire (abt a 50' rectangle) I had been
using inside my apartment. It does have good bandwith - but, as Roy
stated this indicates low radiation resistance.

If you are receiving well, then you are not entirely boxed in - so to
speak (although it does sound like your apartment interior is shielded
by the building structure). You do sound like you have a lot of
height even if it is a air shaft.

Try an end-fed antenna (or work it against the ground of your three
prong plug). For that end-fed antenna, get a long fishing pole with a
reel full of flex-weave wire and a sinker. Mount the pole into a flag
mount so that it points out into the air shaft. Let the sinker loaded
wire drop into the shaft and drive the wire through your tuner.
Naturally, the longer the pole, the better - the more wire, the
better. Who knows, you might be able to tune-up by adjusting the
wire length. You can keep your existing antenna for listening (use a
T/R switch) and you are off and running. Don't worry about anyone
seeing it (especially at night) - they won't.

73's
Richard Clark, KB7QHC