On Nov 21, 4:06*am, "J. Mc Laughlin" wrote:
Spamhog (N1JPR?)

It might be that you are asking for the surge impedance (Z0) of a
transmission line that will minimize the SWR (wrt to the impedance of that
TL) when the TL is connected to your antenna (over a frequency range).
EZNEC offers a straight forward way of answering the question.

Model the antenna and request EZNEC to calculate SWR over the desired
frequency span. *Set the alternate Z0 to something like 200 ohms. *Plot the
SWR on a Smith chart with the alternate Z0 as the center. *Adjust the
alternate Z0 (you do not need to calculate again) until the Smith chart
display is approximately centered and the alternate Z0 will be the desired
Z0 of the transmission line.

It is doubtful that a TL with the desired Z0 exists.

It will be apparent to you that SWR will be high over the wide frequency
range that you mentioned. *As a receiving antenna for use from 3 to 30 MHz,
this fact is of very little importance. *It would be hard to have a receiver
that has such a poor noise-figure as to be adversely affected.

73, * Mac * N8TT

P.S. *The above technique is not applicable for relatively narrow frequency
spans. *Its normal utility is with LPDAs.

J. McLaughlin; *Michigan, USA
Home:



----- Original Message -----
From: "spamhog"
Newsgroups: rec.radio.amateur.antenna
Sent: Tuesday, November 09, 2010 4:48 AM
Subject: feeding random inverted V for RX

K1TTT
just wind a coil choke

Richard
What do you REALLY want?

There is a difference between a 12:1 and a 3:1 mismatch, especially in
a long coax run. What I want is a rough mid-of-the-range estimate of
where impedance in a 50'+50' squat inverted-V dipole across say
3-20MHz.

There is a gazillion description of such antennas fed via 450 ohm
ladder lines, and a few with 600 ohm lines:
- ready made 450 ohm plastic clad line is commercially available
- 600 ohm line can be made with commercially available spacers for
standard wire gauges.

Neither suggests that the corresponding impedance is a good midpoint.
The advantage of ladder lines is extremely low loss, allowing for
massive mismatches without much loss due to the exponentially-
decreasing but still substantial backwave.

Being forced to use a coax, I am looking for a ballpark match not
inspired by the current commercial availability of things I am not
going to include in the design.

The random dipole and the T2FD and related antennas ( like the
ubiquitous 3-wire damped dipole sold by many companies for
professional use) are operationally somewhat similar. *All are
compromise antennas usable in a large spectrum, all are rather funny
in terms of losses, radiation efficiency, and radiation pattern.

The damping resistor is there exclusively to smooth the response over
the spectrum, at a cost. *One could even argue that a balun+coax fed
tuned random dipole and an untuned T2FD of roughly similar size
exhibit similar losses, ^cept one heats the coax more (once the ATU is
peaked), and the other heats the resistor more, which should be handy
in determining deicing strategies. ;-)

There is plenty of literature, including some baseline simulations,
for the T2FD etc.. I haven't found the same for inverted-V random
dipoles. Any pointers? *Have I been googling for the wrong things?

--
J. McLaughlin; *Michigan, USA
Home:

Thank you Mac! All I needed - confirmation that a way exists to find
the ballpark least-bad Z0 given the constraints.

Filippo N1JPR