On Sun, 2010-02-21 at 11:52 -0500, Kris Merschrod wrote:
> I've done years of modeling, building, testing and MFJ analyzing, but then
> again I am a Rural Sociologist! So here goes my 2-cent's worth:
>
> 1) even if the end of the feedline makes your recvr happy at 50 ohms, that
> does not mean that there are NOT SWRs at different points along the feedline
> and radiating points - Therefore loss of ERP.
Technically, there are standing wave ratios everywhere in any feed line.
Sometimes the SWR is low (1:1) when the load matches the characteristic
impedance of the line or is high when the load does not match the
characteristic impedance of the line. And if the line loss is not
significant the SWR is the same at all points along the line. However at
higher frequencies, line loss is more than not significant and the SWR
is higher at the mismatched load than at the transmitter end of the
line.
On a transmission line with a high SWR, there are voltage peaks and
current peaks 90 degrees of phase delay (or one quarter wave in the
line) apart. High current peaks cause increased line loss if the
conductor is not oversized for the current when matched, simply from the
conductor resistance. High voltage peaks can cause increased line loss
from loss in the dielectric. This is not usually significant in decent
coax cables, and is trivial in open wire line with mostly air for
dielectric.
A line with a mismatched load, but a length that produces 50 ohm feed
has mostly the same SWR from end to end. And if the transmitter sees 50
ohms the characteristic impedance of that line can NOT be 50 ohms unless
the line loss is more than 5 to 10 dB for the run. At 432 MHz, 100' of
RG58 has a fine SWR at the feed end, no matter what the load on the
other end because the round trip loss is so high.
>
> 2) The quest for an all band antenna is sort of like the quest for the Holy
> Grail - lost of effort but you really need to know what it looks like and if
> it is resonant at the feed point that only means that the Grail is half
> full.
The venerable T2FD (terminated folded dipole) as sold by B&W is a leaky
dummy load with radiated signal sometimes 20 dB below a resonant dipole
cut for the frequency. But it needs no tuning over 3-30 MHz. A good
receiver probably can stand the 20 dB loss and still have reception
limited by atmospheric noise below 14 MHz, but a communications path
can't always accept the radiation loss. Two EOCs each with the B&W
trying to talk to each other can loose as much as 40 dB and tough HF
circuits can't stand that much loss and have communications.
>
> 3) The other half to fill the Grail are the lobes, hence modeling is
> necessary (EZNEC is my favorite). That means taking into consideration the
> height to boot.
There are definite trade offs in pattern vs wire length. Many ham
antennas are accepted if they radiate in any direction and hear any
signals. A commercial or military circuit that needs frequency agility
to make use of the MUF needs an antenna with consistent directivity and
there's where the log periodic is most commonly used, BUT it takes up a
lot more space than a center fed wire. Paralleled dipoles or trap
dipoles can fill multiband needs for the ham station that hasn't gone to
a tower and a stack of yagis.
>
> 4) finally, hang it up and test it on the air. That moment is analogous to
> sipping from the Grail.
Any hunk of wire will allow contacts under good conditions. It helps to
have a tuner for the solid state transmitter. No antenna at all because
the "optimum" antenna hasn't been found and erected, means no contacts
at all can be made. Toss something up, make contacts, improve it or add
more later.
>
> 73,
>
> Kris (KA2OIG)
>
>
>
73, Jerry, K0CQ
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