[Lowfer] Tuned Active Whips

[wa3tts at verizon.net]

Jay, Les & All:  Having a QTH with 50KW AM stations at 1020 and 1080 kHz 4 and 5 miles distant, and using Jay's single J310 e-probe design, I found the overload solution was an LC shunt on the input across the probe to ground.  I started out with a 330uH inductor and 68 pf silver mica, but that was too broad and it lowered the 630m gain somewhat. Using 470uh and about 50pf with a 10~60pf trimmer cap worked much better to place a null at 1040 kHz.  The result nulled both the 1020 and 1080 kHz by 40 dB or so.  I also noticed the higher Q  470uH and ~50pf combination was more sensitive to hand capacitance when tuning.  To test the LC shunt effectiveness on the input of the e-probe, I feed my signal gnerator into my nearby HF vertical ( and manually swept the signal generator across the AM band
https://www.dropbox.com/s/lwhyh7j5zujjoah/eprsweep.jpg?dl=0
Note there is an external 10dB pad on the input to the HP141T. Without the LC shunt on the e-probe input, the 1020 and 1080 kHz signals both overloaded the HP-141T.
You can see the 1020 and 1080 kHz signals attenuated and the signal generator sweep. The molded chokes have low Q,but the response was adequate for solving my overload problem.  Even less "C" (30 or 40 pf) and more L would likely be desireable to limit LF signal coupling loss in this high Z circuit.  But the compromise issue I was concerned about is temperature effects on the LC shunt tuning. With this LC shunt arrangement, when WH2XND was testing at 75 kHz wspr2 (winter LF season), this e-probe was usually better than my EWE antennas at the time using simple resistive terminations on the EWEs. 
Recently I upgraded to RC terminations and improved CM feed line chokes on the EWEs and the LF/MF patterns are noticeably improved, but that is a topic for another day. It will be interesting to see how the improved EWEs perform this winter vs the W1VD e-probe and LC shunt combination.
Tnx for the bandwidth, 73, Mike wa3tts


-----Original Message-----
From: jrusgrove at comcast.net <jrusgrove at comcast.net>
To: Discussion of the Lowfer (US, European, & UK) and MedFer bands <lowfer at mailman.qth.net>
Sent: Sat, Aug 24, 2019 8:16 pm
Subject: Re: [Lowfer] Tuned Active Whips

Les

A 'tuned' active whip would only be beneficial if one were in a location where strong enough signals were present to overload the active device. If one were in that circumstance it wouldn't seem prudent to use varactor(s) in the tuning network due to their poor performance in the presence of strong signals. 

Jay W1VD