>From: Bogyo Istvan <Istvan.Bogyo@mdc.hu>
To: <topband@contesting.com>
>Date: Wed, 8 Jan 1997 12:28:36 +0100
>
>Dear Fellow LowBanders,
>
>In addition to the beverage sysytem mentioned in another message, I have
>already erected a 27 m vertical for transmitting on TopBand. Again, time
>is short, I would like to see your input, HOW SHOULD I MATCH the
>vertical to 50 ohm coax (RG213). The lower 15 meters of the antenna is
>madeof 2,5" aluminum tubing, the upper 12 meters is 1.75" tubing. It has
>48 quarterwave radials soldered to a central copper plate and several
>grounding rods, the matching network will be in a copper box soldered to
>this plate as well, so I assume the losses are quite low, but I have no
>idea what the radiation resistance can be. I have no antenna scope, and
>probably do not have enough time to make one..
>
>So please advice, what simple matching circuitry should I use?
>
>73 es DX
>
>Steve HA0DU
>
Steve,
I think I can get you into the ballpark for matching that antenna fairly
easily. You should not need any exotic instrumentation to get it done and
this antenna should work quite well for you.
Some quick BOTE calculations show that you will be looking to match a base
impedance of approximately 12 to 13 ohms. So you will need a reactance in
series with the radiator that results in a total unmatched reactance at the
base of the radiator of about -21 ohms. For the radiator you described at
1.840 MHz, this will require an inductor of approximately 15 uH inductance.
Don't skimp on this coil. Make it from 6mm or larger copper tubing with a
winding radius of at least 62mm. Space the turns apart by least the
diameter of the tubing.
To match the radiator system with its series loading coil up to 50 ohms,
you will need a shunt reactance of approximately 29 ohms. IMHO, the best
way to make this shunt reactance element is to use a piece of shorted coax.
You will need 30 electrical degrees of 50 ohm line. Use the largest line
you can find. For example RG213 or RG8 with a velocity factor of 0.66 will
require a length of 8.96 meters. Short one end and connect the other end
across the antenna feedline center to center and shield to shield. This
stub can be routed anywhere along the ground. I usually tape it to the
feedline in a few places. You could also use a coil for this reactance. It
should be about 2.5 uH. If you choose to use a shunt coil, again - don't
skimp.
To operate at other frequencies you have a couple of options. The 3:1 VSWR
bandwidth should be about 80 KHz. So you could use a tuner or the PA's
output network (if it can deal with 3:1) to cover from 1.8 to 1.88 MHz.
Alternatively, you could make the series coil tapped or adjustable to cover
from 10 uH to 17 uH and you could then set the 1:1 (50+j0) resonant point
anywhere between 1.8 and 2.0 MHz.
I hope you find this helpful.
73, Eric N7CL
--
------------------------------------------------------
Eric Gustafson N7CL | The mountains are high and
6730 S. Old Spanish Trail | the Emperor is far away.
Tucson, AZ 85747-9498 |
| You can't work 'em
INTERNET: n7cl@mmsi.com | if you can't hear 'em.
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