RB wrote:
Some questions about a 140' dipole fed with ladderline from an unbalanced
tuner to work 160-10m:

1. Does it matter if the balun is placed on the tuner output, or at the
dipole center feedpoint?

A balun placed at the dipole center feedpoint has BALanced
ladder-line on one side and the BALanced dipole on the other,
both of which are balanced. So why on earth would anyone attempt
to install a BALanced to UNbalanced device at a point where only
balanced environments exist?

2. Which type of balun (voltage/current) would be best for
centerfeedpoint
use?

What you need is a BALBAL. Good luck on finding one at a decent
price that works with the impedances you will encounter.

4. What ratio balun would be optimal for either application?

A Balun should only be installed at a BALanced to UNbalanced
discontinuity. Forget about trying to install them anywhere
else. Most hams who understand what they are doing use a 1:1
choke-balun at the BALanced to UNbalanced point. Take a look
at my 130 ft. all-HF-band dipole (click below)
--
73, Cecil http://www.qsl.net/w5dxp/notuner.htm

Oooopss----I meant coax fed.

On Mon, 5 Dec 2005 14:02:03 -0600, "RB"
wrote:

Oooopss----I meant coax fed.


Did you really?

It is unlikely that your proposed antenna system (140' dipole + bal un
+ coax) feed will provide acceptable losses on many bands, possibly
only one band.

You have given almost no detail of the antenna (and a key element of
the detail was in error), so it is hard to give more a more exact
information.

If you hadn't mentioned the feed line type, readers would probably
have sensibly assumed open wire feed as it is much more likely to be
feasible than coax feed of such a multiband dipole.

The article at www.vk1od.net/LOLL/index.htm shows in Fig 1 and Fig 1a,
antenna system losses for an antenna system comprising a low 66'
dipole and coax feed over HF. Your 140' (I am not sure why you didn't
nominate 133' which would be resonant on 80m for simplified / lower
loss coax feed) would be very roughly a frequency scaled version of
the same thing.

Owen
--

On Mon, 5 Dec 2005 14:02:03 -0600, "RB"
wrote:

Oooopss----I meant coax fed.


If you're really going to feed the antenna with coax, why do you need
a balun? Just run the coax straight to your tuner coax connection.

You're losses on various bands may be pretty high with coax. That's
why most all-banders are fed with ladderline. Losses are lower. But
that's your choice.

Bob
k5qwg

Bob Miller wrote:
You're losses on various bands may be pretty high with coax. That's
why most all-banders are fed with ladderline. Losses are lower. But
that's your choice.

Owen has a feedline loss calculator at: http://www.vk1od.net/tl/tllce.php
EZNEC (including the free demo version at www.eznec.com) can be used
to estimate the SWR to plug into the calculator.

For instance, 100 feet of RG-58a used on 14 MHz with an average SWR of 5:1,
loses ~4.7 dB, or ~66%.

To get the same loss with Wireman #554 ladder-line, the average SWR could
be allowed to equal 53:1.
--
73, Cecil http://www.qsl.net/w5dxp

Bob Miller wrote:

On Mon, 5 Dec 2005 14:02:03 -0600, "RB"
wrote:


Oooopss----I meant coax fed.



If you're really going to feed the antenna with coax, why do you need
a balun? Just run the coax straight to your tuner coax connection.

Sometimes an antenna needs matched to the coax line. An extreme example
is an OCF dipole, which can operate on several bands, but has an
impedence of several hundred ohms. the 4:1 or 6:1 balun will match that
with the 50 ohm coax.


- 73 de Mike KB3EIA -