Topband: BCI chase update

[Tim Duffy K3LR]

Hi Paul:

I have tried and tested many commercial and homebrew preamps for use on the
TopBand (including those you talk about below). Although all of the preamps
work - some work better than others. 

The W8JI design (that DX Engineering sells) has the best noise figure and
overload handling capability of any that were measured. The highest
performance from this preamp is achieved with a 18 VDC power source. I use
this pre amp exclusively with my TopBand RX antennas.

http://www.dxengineering.com/Parts.asp?ID=210&PLID=107&SecID=32&DeptID=12&Pa
rtNo=DXE-RPA-1


A you mention in your paragraph 3 - To protect front ends - the KD9SV front
end saver works very well. It does exactly what you described in a nice neat
box.

http://johnjeanantiqueradio.com/gary.htm

73!
Tim K3LR



-----Original Message-----
From: topband-bounces at contesting.com [mailto:topband-bounces at contesting.com]
On Behalf Of N1BUG
Sent: Sunday, January 08, 2012 11:46 AM
To: Topband
Subject: Topband: BCI chase update

Prompted by Roger, N1RJ, I decided to do some digging around. Aided 
by isolating specific stages of my receive signal path as much as 
possible, and using precision 3 and 6 dB pads at various points to 
see where they dropped the IMD more than the attenuation of the 
pad(s), I have come up with a list of suspects.

1) The preamp. This was oversight on my part. A review of the 
original construction article clearly states a filter will likely be 
required if using a Beverage. Evidence: I simplified the signal path 
to Beverage>Preamp>Receiver. Placing a 6 dB pad before the preamp 
dropped the IMD level more than 12 dB (may be closer to 18, I did 
not attempt to verify). Placing the 6 dB pad after the preamp 
dropped the IMD products 6 dB.

2) The ICE 196 receiver protector. Evidence: 6 dB pad before the 196 
reduced IMD more than 12 dB. Placing the pad after the 196 reduced 
IMD 6 dB. Bypassing the 196 reduced IMD more than 12 dB.

3) A relay used to switch the preamp in and out. This relay had 
apparently "failed", since replacing it with another of the same 
type made a big difference. This likely explains why my IMD problem 
when using the preamp went from annoying to severe a couple years 
ago. (I am aware relays in a receive signal path can be problematic 
due to insufficient current to clean the contacts).

Based on results of this investigation my proposed plan (once I 
figure out how to fund it) is:

1) Replace the preamp. I'm not in the mood to build another one from 
scratch so I'm leaning heavily toward the Clifton Labs Z10040B.

2) Use a HPF in front of the preamp. Whether actually necessary or 
not it seems like good practice. I am leaning toward the Clifton 
Labs Z10022A, although I am torn between that and the Par 
Electronics BCST-HPF. The latter has more attenuation at the high 
end of the BC band, but not as much at the middle and low end.

3) Get rid of the ICE 196. I've had it with these things. This is 
not the first problem I have had with these devices generating crud. 
I have not as yet come up with a plan I am entirely comfortable with 
for receiver protection in its absence but I am leaning toward using 
a relay to short the receive signal path to ground when 
transmitting. Rather than put an additional relay contact in series 
with the signal path, I am thinking about having a normally open 
contact connected to the signal path, taking it to ground when 
transmitting.

I welcome any comments.

73,
Paul N1BUG

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