[TowerTalk] Balan for long dipole?
Ron Stordahl [N5IN]
Ron Stordahl [N5IN]" <ron.stordahl@digikey.com
Thu, 23 Oct 1997 12:26:18 -0500
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Since I cannot get open wire line into my shack I am considering using a =
run of low loss cable, 7/8 heliax for example (because I have some!) =
about 100 feet to a balan at the tower. The balan would be mounted up =
perhaps 10 feet from the base of the tower and I would run open wire, =
such as the 450 ohm 14 ga full legal limit ++ from Cable X-perts from =
that point up to the antenna feed point at the 80 foot level. I want to =
use the antenna on 160/80/40. =20
Its going to have to be an inverted-V, with the ends connected to sturdy =
trees up about 30 feet. These trees are available 190 feet from the =
tower on each side, so I could make the overall length of the inverted-v =
up to 375 feet long.
I have 2 questions, which are likely dependent upon each other:
1. Should I use a 1:1, a 2:1, a 4:1, a 6:1, a 9:1 a 12:1 balan? All of =
these are available from Amadon, and they claim they will handle full =
power. I wish I could verify that before I blow one up at full power =
and 10:1 swr however!
2. How long should it be. The impedance it presents on each band will =
be highly dependent upon length. The ARRL Antenna handbook has graphs =
of impedance (resistive and reactive) for an example dipole of 100 feet. =
For typical wire this resonates at about 5 Mhz where the impedance is =
about 75 ohms. At 10 Mhz the impedance as I recall was very high, over =
1000 ohms, this is the case where the center is a voltage maximum. As =
the frequency increases the curve wraps back, crossing zero reactance, =
then back again etc., it eventually spirals in at 377 ohms resistive. I =
have scaled that example down in frequency to suit my question. A 375 =
foot long dipole would resonate at about 1.2 Mhz. It looks to me like =
it could be a compromise match at 160 and 80. If I cut it to resonate =
at 160 I believe it would be a bear at 80. I want to pick a length =
which allows me to reasonably match the antenna on 160, 80 and 40. I =
will be using a Nye Viking tuner. =20
Here is my guess: Get a 1:1 balan (voltage balan, essentially a 1:1 =
transformer on a ferrite core) and let the tuner deal with it. Then by =
trial and error trim the length of the antenna so that I can readily =
load it on 160/80/40. BUT maybe those of you with real experience can =
answer this straight away. EZNEC might help too.
Ron N5IN
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<DIV><FONT color=3D#000000 size=3D2>Since I cannot get open wire line =
into my shack=20
I am considering using a run of low loss cable, 7/8 heliax for example =
(because=20
I have some!) about 100 feet to a balan at the tower. The =
balan=20
would be mounted up perhaps 10 feet from the base of the tower and I =
would run=20
open wire, such as the 450 ohm 14 ga full legal limit ++ from Cable =
X-perts from=20
that point up to the antenna feed point at the 80 foot level. I =
want to=20
use the antenna on 160/80/40. </FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Its going to have to be an inverted-V, with the ends =
connected=20
to sturdy trees up about 30 feet. These trees are available 190 =
feet from=20
the tower on each side, so I could make the overall length of the =
inverted-v up=20
to 375 feet long.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>I have 2 questions, which are likely dependent upon =
each=20
other:</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>1. Should I use a 1:1, a 2:1, a 4:1, a 6:1, a =
9:1 a 12:1=20
balan? All of these are available from Amadon, and they claim they =
will=20
handle full power. I wish I could verify that before I blow one up =
at full=20
power and 10:1 swr however!</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>2. How long should it be. The impedance =
it=20
presents on each band will be highly dependent upon length. The =
ARRL=20
Antenna handbook has graphs of impedance (resistive and reactive) for an =
example=20
dipole of 100 feet. For typical wire this resonates at about 5 Mhz =
where=20
the impedance is about 75 ohms. At 10 Mhz the impedance as I =
recall was=20
very high, over 1000 ohms, this is the case where the center is a =
voltage=20
maximum. As the frequency increases the curve wraps back, crossing =
zero=20
reactance, then back again <FONT color=3D#000000>etc.</FONT>, it =
eventually=20
spirals in at 377 ohms resistive. I have scaled that example down =
in=20
frequency to suit my question. A 375 foot long dipole would =
resonate at=20
about 1.2 Mhz. It looks to me like it could be a compromise match =
at 160=20
and 80. If I cut it to resonate at 160 I believe it would be a =
bear at=20
80. I want to pick a length which allows me to reasonably match =
the=20
antenna on 160, 80 and 40. I will be using a Nye Viking =
tuner. =20
</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Here is my guess: Get a 1:1 balan (voltage =
balan, <FONT=20
color=3D#000000>essentially </FONT>a 1:1 transformer on a ferrite core) =
and let=20
the tuner deal with it. Then by trial and error trim the length of the =
antenna=20
so that I can readily load it on 160/80/40. BUT maybe =
those of=20
you with real experience can answer this straight away. EZNEC =
might help=20
too.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Ron N5IN</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2></FONT> </DIV></BODY></HTML>
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