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Old February 16th 06, 12:04 AM posted to rec.radio.amateur.antenna
Owen Duffy
 
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Default 300 ohm folded dipole from ARRL Handbook, early 1990's

On Wed, 15 Feb 2006 22:44:00 GMT, Cecil Moore wrote:


It might be interesting to some to explain how/why this works.

....

Some thoughts on why it might not work to plan, though it should be
close to plan and capable of fine adjustment if needed.

Worth noting that the characteristic impedance of some lines is quite
different to their market labelling, especially 300 ohm and 450 ohm
lines.

In this part of the world, nominal 300 ohm TV ribbon is more like 360.
Wes measured a range of nominal 450 ohm lines to be from 360 to 400
ohms.

Although the calculated answer might be a shunt capacitor of 147pF, in
practice a close value would be used and both the dipole length and
feedline length could be juggled for better match if needed.
Alternatively, a shunt stub could be used and tuned along with the
series line length.

The feed point impedance of the dipole will depend on its environment
to some extent, and may be reactive if formulas are used to cut it to
length.

Of course, it is a one-band matching arrangment.

Owen
--
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Old February 16th 06, 10:11 PM posted to rec.radio.amateur.antenna
Phil
 
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Default 300 ohm folded dipole from ARRL Handbook, early 1990's


"Geoffrey S. Mendelson" wrote in message
...
Hi, the early '90s ARRL handbooks had construction information for a
simple folded dipole made of 300 oHm twinlead. The article was not in
the '96 edition or later.

The antenna was made of 300 ohm twinlead. It was fed in the center by
a 300 ohm twinlead feeder to a matching capacitor which made it aproximate
50 ohms. The capacitor could be either a regular capacitor (I used silver
mica WWII surplus) or a "stub" of more twinlead.

At the end of the feed line, which was part of the antenna, you could
attach it directly to a 50 omh unbalanced transciever or a 50 ohm coax
of any length.

The length of the antenna was one half wavelength without the velocity
factor and then shorted at the wavelength points compensating for
the velocity factor.

For example, if it was 10 meters long for a 20 meter dipole, the shorts
were about 4 meters from the center for twinlead with a velocity factor
of 0.8.

If anyone has a copy of the article and could scan it for me, I'd
appreicate it, or even better yet, a pointer to an online version
of it.

Thanks in advance and 73,

BTW, if you are also in Israel and asking yourself "where does he get
the twinlead"? I don't. I brought it with me when I moved here in '96.

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 IL Fax: 972-2-648-1443 U.S. Voice:
1-215-821-1838
Visit my 'blog at
http://geoffstechno.livejournal.com/


I played a lot with a folded dipole for 14 MHz, both with twin lead and with
two parallel wires about 20 cm apart.

To feed it, I used a simple 75 ohms to 300 ohms Neosid transformer
currently/formerly intended to connect a 75 ohms VHF television set to an
antenna with twin lead. Depending on the country, such a bal/un is called a
"symetriser", a "bazooka", a "pig's nose", etc...

This small pig's nose can cope with 100 watts output on 14 MHz and higher.

I never tried it on lower frequencies. I expect it would get rather warm on
80 m.


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Old February 23rd 06, 02:44 AM posted to rec.radio.amateur.antenna
 
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Default 300 ohm folded dipole from ARRL Handbook, early 1990's

Have you experience running 100w into one of these transformers
yourself? I kinda doubt they would take any watts.

If I understand you correctly you are talking about the little unit
that connects the cable to the old 300ohm input of 1970s type tv.
These units have very small toroids, only maybe a quarter inch in
diameter, that are designed for microvolts of excitation at the bottom
end of TV band - which is somewhere near say 50Mhz. They will not
handle any magnitizing flux and I would bet a watt at 20m would
overheat it.

regards,
Bob
N9NEO

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Old February 23rd 06, 10:05 PM posted to rec.radio.amateur.antenna
Phil
 
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Default 300 ohm folded dipole from ARRL Handbook, early 1990's

wrote in message
oups.com...
Have you experience running 100w into one of these transformers
yourself? I kinda doubt they would take any watts.

If I understand you correctly you are talking about the little unit
that connects the cable to the old 300ohm input of 1970s type tv.
These units have very small toroids, only maybe a quarter inch in
diameter, that are designed for microvolts of excitation at the bottom
end of TV band - which is somewhere near say 50Mhz. They will not
handle any magnitizing flux and I would bet a watt at 20m would
overheat it.

regards,
Bob
N9NEO

Those VHF 75/300 ohms transformers where designed for the 48 to 300 MHz VHF
TV-segment. The neosid core is not that small: it just covers my thumbnail.
It indeed can handle 100 watts on 14 MHz. I tested it thoroughly: a friend
had taken his transceiver to Costa Rica where he was visiting family and I
wanted to raise an antenna very quickly to stay in touch with him. I
suspended a folded dipole under the roof of my house and used the described
VHF bal/un to feed it. We had (almost) daily skeds on 14 MHz. The "almost"
was not due to propagation but had more to do with the fact that the
electricity plant was only up 3 or 4 days a week in Costa Rica at that time.

Later I used the same bal/un with a folded dipole for the 20 m band made
with twin lead.

I never used it on lower frequencies. I am sure that it would get pretty hot
with 100 watts on 80 m.

Kind regards,

Phil


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