It will make a dandy multiband antenna if you can avoid having a quarter
wave long feed line on 80 where the feed Z of the antenna is low and the
feed end of the quarter wave line is very high. Like 450 * 450 / 70 =
2892 ohms. And if you don't run 160 meters. If you can make that feed
line near a half wave long on 80 its a quarter wave on 160, the
transformation that raises high impedance feed problems. On all bands
40m higher frequency the dipole impedance isn't low and the open wire
line is the best feed.
On 4/8/2011 3:56 PM, Mike Bryce wrote:
Okay...
So what I just read from the link you sent, if I take and put up a center fed
Zepp with 66 feet on each side, fed with 450 open line and assume no loss from
the tuner or balun that antenna won't make a even halfway decent multiband
antenna?
Mike
Sent from my iPhone
On Apr 8, 2011, at 4:33 PM, Steve Hunt<steve@karinya.net> wrote:
Mike,
Yes, it's a very popular "Ham Myth" - not helped by various articles in QST.
TLW is the software supplied by ARRL with the Antenna Book. It
calculates the losses and impedance transformations for various types of
feedline, given the load impedance. It typically underestimates losses,
and I would normally use the VK1OD on-line calculator in preference to
it, but that doesn't include a model for 600 Ohm line whereas TLW does.
ARRL also can't compute characteristic impedance of low impedance (under
275 ohms or so) parallel lines correctly and refuses to admit it.
Here are the calculated losses for various types of line - all 110ft
long and at 1.9MHz - when terminated in 11-j930 Ohms:
TLW, ARRL "generic" 600 Ohm line: 2.6dB
VK1OD, Wireman 551 ladderline : 6dB
VK1OD, Wireman 551 ladderline (wet): 8.2dB
Remember this is all at 160m !!!!
If you'd like to know more, take a look at this review of the QST
article "The Lure of Ladder Line":
http://vk1od.net/transmissionline/LOLL/
73,
Steve G3TXQ
The apparent low losses of parallel wire line come from the small
currents of a matched load and neglect radiation from wide spacing. The
losses go way up when the SWR is so high that currents get large at the
low impedance points along the line. And if the software computing
losses doesn't take the location of those high current points into
account it will always give wrong answers because at the high impedance
points the current is even smaller so the resistive losses are tiny. If
it computes based only SWR and the line is only a quarter wave long the
loss computation may be as accurate as picking a random number.
At 6m, 1" spaced open wire radiates quite a bit, even more at higher
frequencies.
On 08/04/2011 20:58, Mike Bryce wrote:
Don't shoot me!
But I always read that open line feed was lossless.
Mike
Sent from my iPhone
On Apr 8, 2011, at 3:42 PM, Steve Hunt<steve@karinya.net> wrote:
73, Jerry, K0CQ
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