Super thanks to everyone who responded to my questions. Not only did I learn
all about the pros and cons of antenna wire, not only did I get some
fill-in-the-gap knowledge about radials, but as an added bonus, I even got some
really great, exciting and unexpected advice about the antenna itself!
Several folks gave some great ideas about how to improve this L. My take-away
from this is that even though the tuner could probably load this antenna, my
angle of elevation would be less than ideal for many bands. So I took some
ideas of several people on this list and this is what I came up with, please
see this diagram:
http://www.hertzmail.com/multiband-L.pdf
The idea is to run 3 wires out of the antenna tuner. One 17â long, one 33â
long and the third 130â long with an 80m trap located at 65â-67â out.
The trap would allow some aesthetic improvement (for the xyl) and keep only one
wire running the length across the front yard. This antenna should not be any
more difficult to erect than the plain L. Need to insert a trap and run a two
more ropes to hoist the 17 and 33â sections, but thatâs it. If this would
be much more beneficial than a plain L, then this would seem to be the way to
go.
So, a few questions!
1. How might I expect such an antenna to perform? Will I get âmore
appropriateâ angles of elevation on the different bands than the original,
single radiator L?
2. Can the 17â and 33â sections run completely next to each other? I am
planning to use insulated wire. Would these need to be spaced in some fashion
or can they just rub up against each other and work? I was only familiar with
the fan concept on dipoles, and didnât realize this was a possibility with
the L. One of the things that always scared me about the fan dipole was the
diagrams always showed the radiators âfanning outâ, not running right next
to each other. That always seemed a headache to mount. If they can all run
next to each other without separation (other than insulation) then this becomes
very easy to erect.
3. The thought was to use a trap between 67â and 130â so I donât have two
wires/ropes running across the front yard, inevitably sagging at different
points and looking ugly (thatâs xyl-speak!) Question is, the 80m trap. Will
that cause any significant degradation or loss? Will this loss occur only for
160 or 80? Or perhaps the other bands? Would there be a noticeable difference
in performance if I used a separate wire for this instead of a trap? Or would
the difference be minor?
4. You will notice that the 160m wire [C+D] at a length of 130â in the
diagram would still leave me with about 48â of rope from the insulator to the
tree. Is there any other approach I could take to make more use of this wire?
One nice gent suggested that I use the entire length for 160m (namely 180â)
and let the tuner do the rest. Pros? Cons? Hate to not make use of 48â !
donât know why, just seems a shame ï
5. One fellow suggested that I should expect to have problems with RFI and
ground loops (ouch!) because my shack is on the 2nd floor. I was worried
enough as it is if I were actually going to be able to create and put up an
antenna like this in the first place. Now I am worried that after all that
trouble, there will still be more hoops to go through to avoid some of this
other nasty stuff. Advice?
6. Tuning the wires. How critical is the placement of the trap? Can the main
tuner at the feedpoint compensate for any misplacement of the trap? Other than
trial and error, is there any more precise way to place the trap? For example
(please donât yell at me, this is strictly an academic thought), could I run
the transmitter at very low power (1 watt, for example) and use some kind of
rod with some kind of meter, move the rod along the length of the wire and
locate some kind of voltage node (either high or low or something) and that
would tell me where to place the trap?
One last comment, the tuner I have was designed for 15' to 35â vertical
whips. I believe that the 50 ohm transmitter is immediately converted to 22.2
ohms using an internal transformer. Then the tuner elements take effect (again,
if interested, see <http://www.torontosurplus.com/com/harris/harris_rf601a.htm>
of particular interest, see figure 4-2) I think this has something to do with
the nominal feed point impedance of a 35â whip on board a ship, but I am not
100% certain. I will point out that Harris made a âlong wire kitâ for this
tuner that was essentially a 50pf (very high voltage) capacitor to be added in
series with the wire. I am too green to fully understand the impact of this
capacitor, but I am guessing it sorta stacked the deck a bit to more easily
accommodate the nominal feed point impedance of a horizontal wire vs the whip.
Reason for all this discussion: Several folks have pointed out (and one nice
gentlemen has experience using this tuner) that its range is limited to 2mhz
(if that low). Question is, can I add a fixed capacitor or inductor to help
the tuner match the impedance encountered at a lower frequency of 160m?
Anyway, thatâs MORE than my fair share of questions for the week!
Thanks again to everyone. This list blows the other antenna related lists away
when it comes to so many people offering so much quality advice.
Eugene
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