On Sat, 18 Mar 2006 10:21:27 -0500, "Basil Burgess"
wrote:

Hello all

I've made a first attempt at installing an antenna. It's a 2 Slinky dipole
strung across my roof. I chose the Slinky dipole because it promised to give
good (if not excellent) results in a relatively short antenna. I strung it

There is nothing in your post to indicate how long or short your
antenna is. Perhaps it is a well known design and I just haven't heard
of it in my limited experience.

You say how difficult the coax was to match with an ATU, that is a big
hint that the coax is operating at high VSWR. You say the SWR was
terrible, what does that mean, do you have the numbers? Coax operated
at high VSWR for significant length is quite lossy.

If the impedance presented to the tuner is extreme, you can expect
excessive losses there too.

The ambient noise on low HF bands is very high compared to the noise
floor in your receiver. An antenna system could be seriously lossy,
and yet still allow you to hear all signals above the noise (though at
reduced S meter deflection). Beware of depending on a simplified
receiver test to infer transmit performance.

Owen
--

Owen, you know as well as I do his SWR meter does not indicate SWR on
the feedline where it might matter. So some of your comments don't
make much sense.
----
Reg.

Reg Edwards wrote:
Owen, you know as well as I do his SWR meter does not indicate SWR on
the feedline where it might matter. So some of your comments don't
make much sense.
----
Reg.


Just my two cents worth

For my first transmission aerial I made a three band trapped dipole of
split figure 8 mains cable(80,40 and 20) and fed it with coax, and
strung the centre about 25 foot in the air and falling to about 10 foot
at the ends. This has enabled me to work Britain, Spain, Italy and the
US with 40 odd watts SSB. SWR is not perfect and perhaps I should prune
it a little. The hardest part was tuning the traps. I believe this
design came from the arrl handbook.

I know that its not perfect and perhaps I should refine it, but for a
starter aerial I have found it very good. Having said that there are
things about it I want to experiment with.

One thing I have found invaluable is more experienced local hams who
will help.

There are people much more knowledgeable about antennas here than me,
but I am very happy with my home-brew antenna.

hth

73

Warren
ZL3LC

On Sat, 18 Mar 2006 20:45:38 +0000 (UTC), "Reg Edwards"
wrote:

Owen, you know as well as I do his SWR meter does not indicate SWR on
the feedline where it might matter. So some of your comments don't
make much sense.

Reg,

If Basil measured the SWR at the ATU end of the RG8/X (either directly
or effectively with the ATU in bypass configuration if that is
possible), that would provide information that could be used to
estimate whether losses in the RG8/X were reasonable, and if
reasonable, how reasonable.

The RG8/X is probably the highest risk of serious loss in the antenna
system.

For clarity, I am not at all interested in the VSWR indicated looking
into the ATU unless the ATU is in the bypass configuration, in which
case the measurement is a valid indicator of the VSWR on the RG8/X.

The line loss of 50' of RG8/X at 3.6MHz with a source end VSWR of 8:1
is around 1dB, and probably quite acceptable. Beyond a source end VSWR
of 20:1, losses increase quickly to infinite loss at source end VSWR
around 28:1. If the indicated reflected *power* at the ATU end of the
coax is less than about 80%, or VSWR less than about 20, coax loss is
probably manageable.

Owen
--

Owen Duffy wrote:
On Sat, 18 Mar 2006 10:21:27 -0500, "Basil Burgess"
I've made a first attempt at installing an antenna. It's a 2 Slinky dipole
strung across my roof. I chose the Slinky dipole because it promised to give
good (if not excellent) results in a relatively short antenna. I strung it

There is nothing in your post to indicate how long or short your
antenna is. Perhaps it is a well known design and I just haven't heard
of it in my limited experience.

Owen and Basil,

The slinky dipole comes from the misplaced notion that packing a few
hundred feet of wire in a ten or twenty foot area makes an antenna a
few hundred feet long.

They totally miss the point of why an antenna radiates and how the
steel spiral affects the efficiency. There isn't any attempt to make
the antenna resoant either!

Overall it is a terrible system, unless you just happen through luck to
use it where the antenna has a low-order resonance and current maximum
at the feedpoint.

You could probably trim the slinky and make it work on a few bands
where it would be OK.

If you have enough room, just put up a dipole. Watch out for these
gimmick antennas. There isn't any magic bullet with antennas.

73 Tom