"starman" wrote in message
...
Dave wrote:
replies interspersed
"starman" wrote in message
Since you're using twinlead as the horizontal antenna element, there
is
no point in keeping the two wires in the twinlead seperated at either
end. The twinlead will act like it's one wire if you connect both of
it's wires together at each end. The near end (both wires in the
twinlead) will go to one wire of the high impedance side of the
matching
transformer.
Um, couldn't I get more signal out of a longer wire? And wouldn't I
have
that if I connected the conductors on the far end, and left one hanging
out
in space on the near end?
The two wires in twinlead are too close together to act as a loop or a
longer antenna wire when you connnect just the far end wires together.
If you want a longer antenna you would have to use a longer length of
twinlead.
I forgot to mention in another post that the high side of a shortwave
balun should be about 500-ohms. This is another reason why the
homemade
transformer is better. It's wound for an impedance ratio of about 9:1.
The TV balun in 300-ohms on the high side.
Okay, but I don't understand why I would want a 500 ohm impedance
matching
transformer on a 300 ohm line. Would the impedance mismatch not result
in
an attenuated signal, or signal loss?
You don't have a 300-ohm line. The charecteristic impedance of a single
wire antenna like the inverted-L or random wire is in the range of
400-600 ohms depending on it's height above ground. You are trying to
match that impedance range to the 75-ohm coax.
For some reason, the fog is clearing today. I now remember reading this
somewhere else, but forgot it. I follow you now.
I figured you were going to use just a horizontal antenna section with
the coax going up to one end. That end is where the matching
transformer
should be.
Gotcha. That's where I was going to put it.
The TV balun should have two wires on one side (300-ohms) that may
look
like a short length of twinlead or they could just be two pigtails.
One
of those wires goes to the end of the antenna, which in your case is
the
two wires of the twinlead connected together at that end. The other
wire
of the 300-ohm side must go to the coax shield. That's why some kind
of
adapter is needed with a grounding screw on the 75-0hm side which has
the threads, so you can connect the remaining 300-ohm wire from the
other side to the coax shield and threads via the screw.
The 300/75 ohm matching transformers I have at this point have pigtails
on
the 300 ohm side, and a shield and center conductor on the 70 ohm side
(a
female F connector.) If I am not mistaken, one of the pigtails
correlates
to the center conductor of the female F connector, and the other
correlates to the shield. In other words, I think it's already set up that
way, I don't actually have to connect to to the shield. Am I wrong here?
That's not the way a balun is made. The two 300-ohm pigtails go to the
high impedance winding on the ferrite core in the balun. This winding is
isolated from the low impedance winding that goes to the 75-ohm female
F-connector. That's why it's a transformer. You have to make that
connection from one of the 300-ohm pigtails to the coax shield on the
female F-connector for the balun to work properly in this case.
Gotcha. I just clicked to these facts a few minutes ago. It's a
transformer, not a direct connection. Duh.
I'll look for such an adapter next time I am at the parts depot, probably
late next week.
The shield of the coax will be grounded at some point near the earth
ground. At the antenna end of the coax, the shield needs to be
connected
to the remaining wire on the 300-ohm side which did not get connected
to
the twinlead. I assuming you are not going to run a seperate ground
wire
(don't) up to the end of the antenna where the coax connects to the
matching transformer.
The shield of the coax will be grounded to the earth ground through the
grounding block, I thought. I am picturing something with a F/F adapter
going through it and bolted to it on either side. This is physically
attached to the grounding rod, through an electrically conductive means.
The coax comes down from the roof and connects to one side of the F/F
adapter with a male F connector, and picks up again on the other side
before going to the radio. The shield of the coax is therefore grounded
through the grounding rod which is sunk seven feet or more into the earth.
That's fine. It's the most common way to ground the shield of the coax.
Maybe it's time for a diagram but I think you're close to
understanding
how to do it.
I think I understand, although we are using somewhat different imagery
to
describe it. Since I am not transmitting I think I will try to hook it
up
and see what happens. I am at first going to try clipping this to my
whip,
just to see if I have any signal enhancement. After that I will look at
taking it through a tuner and into the external antenna input.
Connecting the center wire of the coax to the whip will work but it
negates the purpose of using coax as the lead-in conductor. The proper
method is to install a connector on the coax which will plug into the
external antenna jack on the radio. This way you're connecting the
shield of the coax to the radio's electrical ground system which might
help to reduce noise.
I now understand this, too. I was thinking I would make use of the extra
amplification given the whip's input. May still try it, just to see. I've
got the adapter for the external antenna input though.
It's time to build and see what happens.
I'll let you know what happens. It will probably not be until next weekend
though.
Thanks for all your help.
Dave
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