If you have some 75 ohm coax available you can build up a series matching
section:
153 degrees 50 ohm coax connected to the feedpoint followed by 146.5 degrees
of 75 ohm coax.
You can also make a stub match from all 50 ohm coax:
A length of 88 degrees connected to the antenna and a stub of 71 degrees in
parallel at the end makes 50 +j0
Or:
a length of 50 ohms 130 degrees long shunted by a 106 pF capacitor on the
end also makes 50+j0.
I like the series match because it just replaces feedline that you have to
use anyway to get into the shack. No special Tee connectors are needed, just
normal PL259 and double female connectors to hook everything up.
The above lengths are based on an assumption the feed impedance is 79+j0. If
it is different and you can quantify R+j the series section can be "tweaked"
to compensate and still reach a good match.
There are other solutions as well but some of them will require changing the
size of the loop and who wants to do that?
73/K5GW
In a message dated 10/18/2006 3:03:26 P.M. Central Daylight Time,
philc@texascellnet.com writes:
I hastily threw together a 30 meter Delta Loop this summer in order to get
KH8SI in the log for a new one on CW. Using the formula, the resonant point
came out 10.105 mhz (great, first try!) but the feed-point impedance is 79
ohms.
What is a good method of matching this antenna to 50 ohm coax? I don't want
to use ferrites. I'm using a line-flattener in the shack now, but I robbed
it from another project, and I need to return it.
This antenna is a real "keeper;" I was able to bust the pile-up on 2nd call.
The apex is a 50 feet on an 80 foot Rohn 25 tower.
Thanks in advance for any suggestions.
(((73)))
Phil Clements, K5PC
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