Vertical dipole, helically wound - comments?
 

I have taken two lengths of 1.66" OD (1-1/4" nominally labeled) PVC pipe
("top" is 5 feet long, "bottom" is 10 feet long) and connected them with
a PVC "T" joint.

On the top I have wound 210" long 12 gauge, solid copper wire to cover
about 4.75 feet of the "top" PVC length and done the same on the "bottom".

I feed a length of RG-8 into the bottom of the pipe and out the "T"
joint and connect the center conductor to the top winding and the braid
to the bottom winding.

The whole smash is then mounted vertically in an old umbrella stand.

So I have a vertical dipole with roughly lambda / 4 (for 20m) length of
radiator top and bottom. I was not sure whether to wind the top and
bottom the same way so I "matched" them insofar as they look like the
continuous threads of a screw. Not sure if that is right.

Seems to work well as a receiving antenna, not sure on the transmitting
part as yet.

Comments, especially regarding construction changes and tuning would be
welcome.

Thanks.

73s

Thomas

"Thomas" wrote in message
...
I have taken two lengths of 1.66" OD (1-1/4" nominally labeled) PVC pipe
("top" is 5 feet long, "bottom" is 10 feet long) and connected them with
a PVC "T" joint.

On the top I have wound 210" long 12 gauge, solid copper wire to cover
about 4.75 feet of the "top" PVC length and done the same on the "bottom".

I feed a length of RG-8 into the bottom of the pipe and out the "T"
joint and connect the center conductor to the top winding and the braid
to the bottom winding.

The whole smash is then mounted vertically in an old umbrella stand.

So I have a vertical dipole with roughly lambda / 4 (for 20m) length of
radiator top and bottom. I was not sure whether to wind the top and
bottom the same way so I "matched" them insofar as they look like the
continuous threads of a screw. Not sure if that is right.

Seems to work well as a receiving antenna, not sure on the transmitting
part as yet.

Comments, especially regarding construction changes and tuning would be
welcome.

Thanks.

73s

Thomas

I see 2 problems here Thomas. You are confusing the length of the wire you
wound into inductors as having the same electrical length as a straight
piece- nothing could be further from correct. In fact, the relationship
between the amount of wire in the coils and their equivalent electrical
length is complex and depends on a number of factors including where along
the radiator the inductors are placed.
Secondly, running the coax up inside the PVC will likely result in a large
amount of RF being impressed on the exterior of the coaxial shield.
Visit Reg Edward's site to see how to electrically shorten a radiator with
inductive loading.

Dale W4OP

I think the main problem you'll have is feeding the coax through PVC -
you'll likely get lots of rf onto the braid.
think about feeding through some metal pipe - to protect the coax from
induced rf


"Thomas" wrote in message
...
I have taken two lengths of 1.66" OD (1-1/4" nominally labeled) PVC pipe
("top" is 5 feet long, "bottom" is 10 feet long) and connected them with
a PVC "T" joint.

On the top I have wound 210" long 12 gauge, solid copper wire to cover
about 4.75 feet of the "top" PVC length and done the same on the "bottom".

I feed a length of RG-8 into the bottom of the pipe and out the "T"
joint and connect the center conductor to the top winding and the braid
to the bottom winding.

The whole smash is then mounted vertically in an old umbrella stand.

So I have a vertical dipole with roughly lambda / 4 (for 20m) length of
radiator top and bottom. I was not sure whether to wind the top and
bottom the same way so I "matched" them insofar as they look like the
continuous threads of a screw. Not sure if that is right.

Seems to work well as a receiving antenna, not sure on the transmitting
part as yet.

Comments, especially regarding construction changes and tuning would be
welcome.

Thanks.

73s

Thomas


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"Dale Parfitt" wrote
Visit Reg Edward's site to see how to electrically shorten a radiator with
inductive loading.

===============================

Hi Dale, yo took the words right out of mar mouf.

Program HELICAL deals with a vertical, continuously-loaded, plastic-pipe
antenna, above a ground or above the roof of a vehicle. Because of the
uncertain surroundings the program will put you in the right ball park with
the number of turns on a pipe of given length and diameter. Putting two
antennas back-to-back allows the design of short dipoles without any ground
loss.

Program MIDLOAD deals directly with short, centre-fed, continuously-loaded
1/2-wave dipoles with a variety of feeding arrangements. May be either
vertical or horizontal dipoles versus height above ground.

These antenna programs are most suitable for the 160, 80 meter and perhaps
40m bands where space is at a premium. At the higher frequencies
lump-loading coils can be (or must be) used.

The advantage of continuous helical windings is that THICK, spaced, wire
(copper pipe even) can be used. Thick wire means low loss resistance and
higher radiating efficiency. Otherwise there's no point in using long
helical windings.

There are a variety of my programs which use lumped loading coils some of
which incorporate coil design and feedpoint impedance-matching arrangements.
In the website refer to the one-line descriptions following each clickable
program name.

Incidentally, changing the subject, the reason for using single-turn coils
for magloops is that it allows a VERY thick conductor, a 1" or 2" copper
pipe, to be used. Otherwise there's no point in using a magloop for
transmitting purposes. The magloop is by far most efficient of all the small
(in terms of wavelength) antennas. The disadvantage is its expensive or
heavily-engineered tuning capacitor. Use program MAGLOOP4 for an accurate
performance analysis.

For all readers, download the small, single-file, easy to use, programs in a
few seconds from the following website and run immediately. Get your money
back if dissatisfied!
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Hal Rosser wrote:
I think the main problem you'll have is feeding the coax through PVC -
you'll likely get lots of rf onto the braid.
think about feeding through some metal pipe - to protect the coax from
induced rf

As I understand it, he is feeding the coax through the center of
the bottom helical coil. What's going to protect the coil from
the metal pipe? :-)
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:

"Dale Parfitt" wrote
Visit Reg Edward's site to see how to electrically shorten a radiator with
inductive loading.

Hi Dale, yo took the words right out of mar mouf.

There's a similar thread over on eHam.net.
" Linear loaded mobile antennas"
"Reply by K0BG on August 11, 2004"
"Loading coils DO NOT lengthen an antenna."
Quoted from:
http://www.w8ji.com/mobile_and_loaded_antenna.htm
--
73, Cecil http://www.qsl.net/w5dxp



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I would use the metal pipe as part of the conductor on that leg - it
wouldn't be pretty - metal pipe wound helically
;-)

"Cecil Moore" wrote in message
...
Hal Rosser wrote:
I think the main problem you'll have is feeding the coax through PVC -
you'll likely get lots of rf onto the braid.
think about feeding through some metal pipe - to protect the coax from
induced rf

As I understand it, he is feeding the coax through the center of
the bottom helical coil. What's going to protect the coil from
the metal pipe? :-)
--
73, Cecil http://www.qsl.net/w5dxp



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how would helically wound stack up against one folded back on itself
(elongated 's')? both are linearly loaded yes-no?


"Cecil Moore" wrote in message
...
Reg Edwards wrote:

"Dale Parfitt" wrote
Visit Reg Edward's site to see how to electrically shorten a radiator
with
inductive loading.

Hi Dale, yo took the words right out of mar mouf.

There's a similar thread over on eHam.net.
" Linear loaded mobile antennas"
"Reply by K0BG on August 11, 2004"
"Loading coils DO NOT lengthen an antenna."
Quoted from:
http://www.w8ji.com/mobile_and_loaded_antenna.htm
--
73, Cecil http://www.qsl.net/w5dxp



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