I have a very short space where I need to put up a temporary antenna.
It has to be rolled out during the day and rolled up and hidden at night.

20m would be a good band for me to use, and it will just about fit, so
I was thinking of a 1/4 wavelength radiator with a 1/4 wavelength
counterpoise made out of wire.

Since I have lots of 300 Ohm twinlead just sitting around, I could
make the radiator out of it. I understand that if I were to short both
ends and center feed it, it would be a standard 300 ohm folded dipole.

I think if I were to short the ends and end feed it, it would be a
eaquivalent to a wide(er) 1/4 wavelength end fed antenna and still
need a counterpoise.

What would happen if I were to end feed it as if it were two seperate
wires, either with a balun or without? Would it work? What is this called?

Thanks in advance,

Geoff.



--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

(Geoffrey S. Mendelson) wrote in
:

I have a very short space where I need to put up a temporary antenna.
It has to be rolled out during the day and rolled up and hidden at
night.

20m would be a good band for me to use, and it will just about fit, so
I was thinking of a 1/4 wavelength radiator with a 1/4 wavelength
counterpoise made out of wire.

Since I have lots of 300 Ohm twinlead just sitting around, I could
make the radiator out of it. I understand that if I were to short both
ends and center feed it, it would be a standard 300 ohm folded dipole.

I think if I were to short the ends and end feed it, it would be a
eaquivalent to a wide(er) 1/4 wavelength end fed antenna and still
need a counterpoise.

What would happen if I were to end feed it as if it were two seperate
wires, either with a balun or without? Would it work? What is this
called?

Thanks in advance,

Geoff.




If I understand what you are suggesting it would be a shorted transmission
line, not an antenna. I would look at an end feed half wave antenna, like a
zep or the ones from PAR electronics. I've homebrewed them myself and they
do work.

John Passaneau

Geoff Mendelson wrote:
"I think if I were to short both ends and center feed it, it would be an
equivalent to a wid(er) 1/4 wavelength end fed antenna and will still
need a counterpoise."

No counterpoise needed. Its called a "folded quarter wave dipole". Its
first resonance is due to its 1/2-wave circumference. Its input
resistance is not 300 ohms but 600 ohms. Its gain is 0,5 dB less than
the 1/2-wave dipole. This information is from "TV and Other Receiving
Antennas" by Arnold B. Bailey.

You might want to feed it with 450 ohm ladder line.

Best regards, Richard H. Harrison. KB5WZI

On Mon, 14 Jan 2008 12:49:14 +0000 (UTC), (Geoffrey
S. Mendelson) wrote:

20m would be a good band for me to use, and it will just about fit, so
I was thinking of a 1/4 wavelength radiator with a 1/4 wavelength
counterpoise made out of wire.

Hi Geoff,

I presume by this you mean you have enough, total length, to create a
halfwave dipole for 20M in the conventional sense.

Since I have lots of 300 Ohm twinlead just sitting around, I could
make the radiator out of it. I understand that if I were to short both
ends and center feed it, it would be a standard 300 ohm folded dipole.

To some degree, but that will bring nothing to your advantage.

I think if I were to short the ends and end feed it, it would be a
eaquivalent to a wide(er) 1/4 wavelength end fed antenna and still
need a counterpoise.

Yes, and it brings into doubt my presumption above. It would inform
us all if you gave us the dimensions of your available space.

What would happen if I were to end feed it as if it were two seperate
wires, either with a balun or without? Would it work? What is this called?

An aircooled resistor.

73's
Richard Clark, KB7QHC

Richard Harrison wrote:
No counterpoise needed. Its called a "folded quarter wave dipole". Its
first resonance is due to its 1/2-wave circumference. Its input
resistance is not 300 ohms but 600 ohms. Its gain is 0,5 dB less than
the 1/2-wave dipole. This information is from "TV and Other Receiving
Antennas" by Arnold B. Bailey.

You might want to feed it with 450 ohm ladder line.

Ok, thanks.

Geoff.


--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

Richard Clark wrote:
I presume by this you mean you have enough, total length, to create a
halfwave dipole for 20M in the conventional sense.

No, I don't. I have exactly 25 feet. This needs to include any insulaters
of support ropes at the ends, if I use them.

Yes, and it brings into doubt my presumption above. It would inform
us all if you gave us the dimensions of your available space.

What would happen if I were to end feed it as if it were two seperate
wires, either with a balun or without? Would it work? What is this called?

An aircooled resistor.

Oh, thanks.

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

On Mon, 14 Jan 2008 15:49:10 +0000 (UTC), (Geoffrey
S. Mendelson) wrote:

Richard Clark wrote:
I presume by this you mean you have enough, total length, to create a
halfwave dipole for 20M in the conventional sense.

No, I don't. I have exactly 25 feet. This needs to include any insulaters
of support ropes at the ends, if I use them.

Hi Geoff,

Put up 3 to 5 sections of twinlead (with the wires shorted at both
ends) on each side, fanned out at the far ends across 3 to 5 feet
(looks like a bowtie). Feed at the apex of each triangular half, that
meet in the middle. Choke the cable feed.

2:1 SWR bandwidth should be at least 1 MHz wide.

73's
Richard Clark, KB7QHC

20m would be a good band for me to use, and it will just about fit, so
I was thinking of a 1/4 wavelength radiator with a 1/4 wavelength

You can easily make antennas that are hidden. For example, get some of that
aluminium tape they used on windows, then use it to make a loop on a large
window. The alarm tapes also have contact pads that you can use to make
contacts with them to feed the antenna. Such an antenna will work well for
transmitting and receiving. You can change the direction by opening the
window.

You could fasten the antenna to a packinmg crate lid, or even use the metal
band around the crate, as I did. Hang it on the wall and put a dartboard on
the other side. When finished just turn it over, or even leave it
permamently hidden behind the dartboard. Put a lamp over the board to excuse
the cable. http://web.telia.com/~u85920178/antennas/crate.htm

You could fix the aluminum tape to a carpet, then roll it up when finished.

Do you have the capability of adding a new plastic drainpipe? Looks good
from outside, but can hide an antenna inside ;-)

----------------

If you are having trouble with neighbours, then make a wooden antenna using
brush-handles, or even a fishing pole. Let the neighbours see it and make a
fool of themselves when they accuse you of disturbance to the TV sets. Then
you can use the real antenna another day without fear. ;-)

Richard Harrison wrote:
Geoff Mendelson wrote:
"I think if I were to short both ends and center feed it, it would be an
equivalent to a wid(er) 1/4 wavelength end fed antenna and will still
need a counterpoise."

No counterpoise needed. Its called a "folded quarter wave dipole". Its
first resonance is due to its 1/2-wave circumference. Its input
resistance is not 300 ohms but 600 ohms. Its gain is 0,5 dB less than
the 1/2-wave dipole. This information is from "TV and Other Receiving
Antennas" by Arnold B. Bailey.

You might want to feed it with 450 ohm ladder line.

Best regards, Richard H. Harrison. KB5WZI

In my copy of Bailey (1950), he says (p. 415) that the feedpoint
resistance of this antenna at resonance is very high, its value
depending on the conductor diameter and spacing. He shows one example
having a resistance of 4160 ohms, and states (p. 416) that

"Thus we see that at first resonance the [quarter wavelength] folded
dipole exhibits the qualities of a shunt-tuned circuit and has a
terminal resistance value Ra of high magnitude. This precludes its
general use where a normal transmission line is employed, unless the
rods are made very thick."

Even if you did manage to reasonably match the antenna, the bandwidth
would be very narrow and the loss potentially high.

Sorry, as Robert Heinlein said, TANSTAAFL - There Ain't No Such Thing As
A Free Lunch.

Roy Lewallen, W7EL

"Harry - SM0VPO" wrote in
:

If you are having trouble with neighbours, then make a wooden antenna
using brush-handles, or even a fishing pole. Let the neighbours see it
and make a fool of themselves when they accuse you of disturbance to
the TV sets. Then you can use the real antenna another day without
fear. ;-)


Or get a Ham buddy to make a QSO on your station while you are over
at the neighbors! "How can it be my station when I'm standing here in
front of you?? hehe

- 73 de Mike N3LI -