Topband: More Inv-L matching issues....Capacitor Voltage Analysis

[Paul Dulaff]

Dan

Quick NEC analysis of the 160 meter inverted L antenna:

Total length of 167 feet.

SWR to 1.2 to 1, assumed center freq of 1.85. 2 to 1 SWR points are at 1.80 and 1.90 Mhz.

Reactance is 250 ohms, Capacitor value is 344 pF.

Voltage across the cap is 1250 volts peak at 500 watts input power to the antenna.

Checking the spacing chart, an air gap of 1.0 mm (0.040 in.) will work at this voltage.

If possible, take a look at the system a see how close this analysis is.

Best 73's

HNY

Paul - WB2NMI

----- Original Message -----
From: Paul Dulaff <pdulaff at embarqmail.com>
To: N8DCJ at YAHOO.COM
Cc: topband at contesting.com
Sent: Mon, 02 Jan 2012 16:03:53 -0500 (EST)
Subject: Re: Topband: Fwd: Re: More Inv-L matching issues.... BIGGER ISSUE

Dan

Its good that the list has supplied useful suggestions for troubleshooting. To reduce the risk to the RCS-10, you may want to bypass it and connect directly to the inverted-L for this test. I expect that your plan is to load up the 374A with no more than a few watts output, just enough to get an SWR reading. If the amplifier is operating on the second harmonic, the SWR to the antenna will be remarkably BAD.
 
While the RCS-10 was the failure point, without it the failure point could become the amplifier tank circuit since I beleive that the system is operating at 2X freq., thus the load (antenna) looks like a very high impedance, the transmission line is looking into an open. The resulting high voltages then took out the RCS-10 antenna switch.

If possible, you may want to look at the tuning capacitor at the antenna. It may have arced over, especially if we are correct about the second harmonic operation, the capacitor has then seem some high voltage. Usually once a capacitor has arced over, the next application of power will arc the capacitor at the same spot and require less voltage than the first arc event.

Let us know what air gap the capacitor has and your estimate of amount of capacitance engaged. From this info, the antenna system can be approximately modeled to calculate the voltage across the capacitor.

FWIW, I run my inverted-L without a capacitor. I pruned the length of the antenna to obtain best match. I can then run full power without the concern of damaging the tuning capacitor.

Good luck with the station.

HNY

Paul - WB2NMI

----- Original Message -----
From: Dan Bookwalter <n8dcj at yahoo.com>
To: rstealey at hotmail.com, pdulaff at embarqmail.com
Cc: topband at contesting.com
Sent: Mon, 02 Jan 2012 13:15:06 -0500 (EST)
Subject: Re: Topband: Fwd: Re: More Inv-L matching issues.... BIGGER ISSUE






Paul
I have not yet been able to try anything :-( 
I hope by Wednesday to start working on it , my plan is to ground the RCS-10 , and try retuning the amplifier , if I burn out another relay my plan is to totally rework the feedpoint of the antenna , move the switch to the otherside of the tower so there is about 4-5 feet from the Inv-L. I can't afford to burn up anything else.
I will report the results.
Dan N8DCJ




Sent from Yahoo! Mail on Android

From: Paul Dulaff ; 
To: Rick Stealey ; 
Cc: ; 
Subject: Re: Topband: Fwd: Re: More Inv-L matching issues.... BIGGER ISSUE 
Sent: Mon, Jan 2, 2012 4:27:28 PM 
Has anyone heard from Dan ?
Did he get the problem solved ?
Paul - WB2NMI
----- Original Message -----


From: Rick Stealey 


To: topband at contesting.com


Sent: Mon, 02 Jan 2012 07:39:55 -0500 (EST)


Subject: Re: Topband: Fwd: Re: More Inv-L matching issues.... BIGGER ISSUE



> it may be possible that the amp is tuned up for output on 80 meters. 
Excellent suggestion.  Since the amp has an swr meter you can easily verify if it is tuned to 80 meters.


Test it at the lowest power possible.

                        


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UR RST IS ... ... ..9 QSB QSB - hw? BK
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UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK