Very interesting! I have this very problem with WINX 1600 KHz AM located
just a mile from me.
In fact, I noticed the intermod last Friday morning..And K3GEG on the 147.00
repeater gave the conclusive analysis of the intermod I was getting. I was
hearing many USA guys calling on 1908 when I was trying to copy the JAs.
Actually, they were on 3508 calling AL7EL/KH9. Per K3GEG, 3508 - 1600 = 1908.
I had previously, many times over the past 6 months since I installed it,
noticed that often my NE (CONTEST) EWE wouldn't work. I'd go out and check
it, remove the coax connection, jiggle the connections, etc looking for an
intermittant. Then I'd turn the power off and it'd work next time I turned
it on. I thought the ZJ box was intermittant.!! That EWE points right at
WINX!!
In actuality, I had an R&D department (W6GO and K2ZJ) working on the problem
all the time!! Thanks, Jay & John!
I'm going to try this "fix".
Bill, N3RR
P.S. John, K2ZJ, can you ship me a couple of MARS 3 devices? As you know,
I have seven of your boxes. Thanks. Bill Hider, 10241 Lakewood Dr.,
Rockville, Md. 20850-3661.
Bill
>From: w6go@netcom.com (Jay O'Brien - W6GO)
>Subject: TopBand: ZJ Beverage box modifications
>To: topband@animas.frontier.net
>Cc: zjelect@aol.com
>Content-Length: 7265
>Sender: owner-topband@frontier.net
>Precedence: first-class
>Reply-To: w6go@netcom.com (Jay O'Brien - W6GO)
>
>PREFACE from W6GO: I have sent a previous edition of this message
>several times to the Topband reflector for distribution and it
>didn't get distributed. I was using a word which I understand the
>list server (MajorDomo) doesn't like, and I added ** to the
>beginning and end of the word to try to fool the good Major. He is
>apparently not fooled easily. This has been edited to remove all
>occurrences of that word (don't ask!). If I don't see this come out
>of the Topband reflector, I'll post it to the contest and DX
>reflectors, as I believe there are some who can use the information
>below.
>
>ZJ Beverage box modifications:
>==============================
>
> Do you use a ZJ Beverage Box? This may be of interest to you if
>you are experiencing intermodulation products or overload.
>
> I installed a 700' beverage which points almost directly at KFBK,
>a 50kW station on 1530 kHz which is 9 miles away. KFBK has a
>broadside array which supposedly has 6db gain in my direction. KBFK
>is very strong here! I measure -1dbm from KFBK off the beverage (.6v
>RMS across 390 ohms). KFBK is strong enough here that if I put a
>small (#222) flashlight bulb across my transmit antenna feedline it
>glows dimly from the KFBK energy.
>
> In checking the new ZJ box I bought for this beverage, I noted that
>with a 3 foot cable between the beverage box and the power insertion
>box, that about one time out of 5 when I turned on the power that
>the gain was down about 10db and unstable. With 50' of coaxial
>line, the problem didn't surface. With a spectrum analyzer, I found
>that when the gain was down, the beverage box was oscillating at
>1.25GHz (1250 MHz) in the 23cm ham band! The problem seemed to go
>away with the longer cable, so I ignored it and installed the ZJ
>box. The beverage was unusable. 160 was full of "junk" from KFBK
>and other sources. That's when I measured the KFBK level and
>brought the box back to the bench.
>
> Watching the box with a spectrum analyzer and tracking generator,
>I inserted a 1530 KHZ signal at 0.6V RMS. The 1.25 GHz oscillation
>started again! Once started, the only way to stop the oscillation
>was to remove power. The oscillation was kicked off with 0.2V RMS
>at 1530 KHz, and it was kicked off with 20db less signal (0.02V RMS)
>if the signal was inserted at 1800 kHz. The internal input filter
>in the Beverage box exhibits 20 db loss at 1530 KHz, so that made
>sense. When the oscillation was present, the 160M band was covered
>with "junk".
>
> Thanks to advice from Dave Leeson, W6QHS, of California Microwave
>fame, I cured the oscillation. There are certainly other
>combinations and values which would work, but here's what worked for
>me. On the ZJ pc board (dated 7/28/94) about 1/2 inch below the "Z"
>in the "ZJ ELECTRONICS" is a small four-terminal device, about 1/4
>the size of an asprin tablet. That's the amplifier chip. Two of
>the leads connect to ground. The lead closest to the "Z" is the
>input, and the other ungrounded lead is the output.
>
> Using minimum lead length, solder 100 ohm 1/4 w carbon film
>resistors to the input and output solder pads. To the free end of
>the new resistor nearest the ZJ marking, again using minimum lead
>lengths, solder one side of a ceramic 24 pf capacitor. Solder the
>other side of the capacitor to the ground plane near the second "E"
>in the ZJ ELECTRONICS marking. To the unconnected end of the other
>100 ohm resistor, again with minimum lead lengths, solder two 24 pf
>capacitors in parallel between the resistor and ground, soldering
>one 24 pf capacitor to the ground plane on each side of the
>four-terminal device. I used 24pf because I have a box full of
>them, 22pf standard value capacitors will work the same, I'm sure.
>
> I have made this modification to three of the ZJ boxes, all of which
>exhibited the same oscillation before modification. Only one of
>them sustained oscillation after the big input signal was removed,
>but all three oscillated when a "big" signal was present at the
>input of the beverage box.
>
> This problem probably won't show up for anyone who doesn't have BIG
>signals heard by their beverage. However, IF you do, consider this
>modification. Who would ever suspect a 160 meter preamp to oscillate
>in the 23cm ham band?
>
> After the cure of the 23cm oscillation, I turned my attention to the
>remaining IM problems. I built a parallel resonant trap for 1530 KHz
>with a 3900 pf capacitor and small toroid which is in series with the
>beverage at the ZJ box. This reduced KFBK 10 db with no effect at
>1800 KHz, and overload from KFBK is now not a problem alone.
>However, when another nearby 160M transmitter is on, I again
>experience intermod which is keyed on and off by the 160M signal.
>
> John, K2ZJ, FedExed me some new amplifier chips (MAR3) which have
>a higher 3rd order intercept point. I have installed these in two of
>the beverage boxes (time for tweezers and magnifying glass!) with
>great results. I also had to add a resistor to raise the voltage a
>bit to the chips. I spent a lot of time determining exactly what to
>measure and more time confirming the dc level to feed to the chip.
>I'll skip all that and jump to the bottom line. Here's my test data
>supporting the IM improvement:
>
> I connected the beverage box output to a 3db hybrid splitter which
>connected to my receiver and to a HP spectrum analyzer. Using the
>HP, I measured the signal level from KFBK on 1530 kHz. Then I
>transmitted on 1815 kHz with a 3 watt carrier on my vertical
>transmit antenna and measured the 1815 kHz signal level on the HP.
>
> I then measured the signal level of the 2100 kHz 3rd order product
>(2*1815kHz minus 1530 kHz). Of course this test includes all of my
>antennas and the possible problems with them, but it IS what I have
>to live with!
>
> With either the old (MAR6) or new (MAR3) devices, my 1815 kHz
>signal measured -12 dbm. KFBK measured -20 to -22 dbm (note the
>filter in the ZJ beverage box knocks KFBK down 20 db from 1800
>kHz). With the old (MAR6) device, the 2100 kHz IM product measured
>-27dbm. With the new (MAR3) device, the 2100 kHz IM product
>measured -40dbm, an improvement of 13 db. At the same time most of
>the "junk" heard between 1800 and 1850 mHz went away. The MAR6 is
>supposed to have slightly less gain then the MAR3, but I don't
>notice any difference.
>
> I haven't had real on-the-air confirmation of the reduction of IM
>products when a nearby 160M transmitter is on, but based on these
>measurements I feel the problem is now gone. Besides, it "sounds
>better"!
>
> John, K2XJ, has verified the 1.25 gHz oscillation problem on other
>units and will modify future units to prevent the problem in high RF
>environments such as mine. John will also provide the MAR3 in new
>beverage boxes for anyone who may need them due to a high RF
>environment.
>
> The ZJ box is well built and continues to be my choice for
>beverages. I am pleased with the support I've received from John.
>
>73, Jay
> w6go@netcom.com
>
>PS: Next: A band-stop filter for 1530 kHz designed for the input
>impedance of the ZJ beverage box, using W3NQN's design in his
>November 1995 QEX article. It can't hurt to reduce KFBK even more!
>
>cc:zjelect@aol.com
>
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