Hi Cecil,

That's how I solved my problem:  I made 1', 2', 4', 8' sections of ladder line and used them in incremental combinations until the solution was found. 

I wish I'd thought of using automotive crimp connectors though - would have made the job far quicker and simpler.  Excellent idea, thanks.

73,
John
VK7JB

Hi Group,

Thanks to everyone for their input, on and off the list.  I've learnt a lot.

I'd like to clarify a few aspects of my initial posting, because I'm not sure I described my setup very well:
Firstly, my problem related to stray RF in the shack at certain ladder line lengths and not to any inability of my ATU to match the antenna system to 50 ohms.  In fact, at all ladder line lengths, including those where RFI was a problem, the internal ATU in my K3 and my manual, Palstar tuner were both able to find easy matches on all bands. Secondly, the balun I mention at the transition from ladder line to my short run of coax is a 1:1 common mode choke (50 ohm coax wound through a toroid stack)  and I'd expect any impedance transformation is likely to be very small.  Finally, I found my solution empirically - through the incremental addition of one foot lengths of ladder line until I found a length at which the EMC issues resolved on all bands.  I can't see any simple relationship between my ultimately successful length  of ladder line and the published "rules of thumb" for the use of ladder line in multiband antenna systems such as mine.

So what I've learnt is this:  such multi-band antenna systems are complex, with multiple interactive elements. To arrive at a "friendly" feedline length, it's important to know the locations of the differential mode current/voltage maxima and minima and  also the locations of the common mode current maxima along the feedline. To complicate matters, the velocity factor for energy transfer along the line in differential and common modes may well differ and then, placement of any balun in the system will influence the common mode current distribution along the feedline too, depending on the specifics of the design and implementation of the balun in the system.

I didn't really appreciate the complexity before this and all I can say is that I'm pleased that I found my "successful" length of feedline by experimentation. Given the multiple variables, I'm not sure how I would have confidently or accurately modeled or predicted that length prior to taking out my wire cutters and cutting the ladder line to its final length.  Maybe that's a reflection of my very limited modelling skills.  But, I'm an average ham and were I to approach this situation again, I would do it in the same way - by experimentation, a foot at a time.  In my case, the difference between obtrusive RFI on 2 bands and perfect audio and an EMC free shack was just 2 feet of ladderline. 

Knowing what I know now, I can't see where I'd start to be able to predict that "magic length" with confidence. 

Thank you all for your input.  It's been an interesting learning curve.

73,
John
VK7JB

Hello Cecil,

Thank you.  This makes sense to me - changing the ladderline length is shifting the position of the current minimum/voltage maximum in relation to the balun and, I guess, also in relation to the RF sensitive equipment and connections at my operating position.

 So, out of interest, how do I calculate or measure the position of the current maxima and minima along my feedline?  I've seen your DOS-based calculators, but I understand they relate to dipoles. How do I apply them to my loop feedline?

73,
John
VK7JB

Hello Group,

I'm looking to understand something I've observed - can anyone help?

I run an 80m loop as a multiband antenna and feed it with 300ohm ladderline. Outside the shack, I transition the balanced line to a 10 foot run of RG213 via a Balun Designs, 1:1, toroidal choke balun. The coax then comes through the wall into my unbalanced Palstar manual tuner.

Recently, after changing the loop orientation,  I shortened the run of ladderline to make it a more convenient length and to avoid too much excess.  After shortening the line to 52 feet, I noticed that for the first time, I had RF in the shack with distorted transmitted SSB audio on 40m only. I increased the ladderline length to 60 feet and the RF in the audio resolved on 40m, but was now present on 20m. After changing the ladderline length to 54 feet, there is no stray RF on either 20 or 40m and the audio is clean.

Apart from the changes in ladderline length, I made no other changes to the antenna system. During all of this, I used an MFJ854 clamp on current meter to look for common mode current on the coax in the shack, but none was detectable on the most sensitive scale, despite the obvious effect of stray RF on my transmitted signal. I could detect no common mode current on my power supply cabling either.

I'd like to understand why this RF ingress should be so sensitively dependent on the ladderline length? Could this still be due to common mode current, even if my clamp on meter registers none?

Any explanations welcome. I'm keen to learn from this.

73
John
VK7JB

I had DX fun with an OCF dipole for several years. I used 4:1 current baluns of transmission line design, some commercial and others home brew, including the DX engineering and Balun Designs models. What I learnt from observation is this:  the 4:1 balun at the feedpoint may perform an effective impedance transformation, but it's common mode impedance is not sufficient to effectively decouple the coax feedline in this unbalanced antenna design.

To keep common mode current low and minimize RFI, I always found it necessary to add a toroidal common mode choke, either directly beneath the 4:1 balun at the feedpoint, or further along the coax towards the shack at a convenient point. Once I twigged to this, my stray RF was gone.

I do not believe that 4:1 TL balun designs can serve as effective common mode chokes in this application.

Just my 2c!

73,
John
VK7JB

Hi John,

I run an 80m loop as a multiband antenna. I've fed it over the years with various combinations of open wire line, commercial ladder line and coax.

To reassure myself about the likely losses of the various combinations, I have found this online tool very helpful and illuminating.

http://vk1od.net/calc/tl/tllc.php

73,
John
VK7JB

Hello Group,

I’m hoping someone can help me with a couple of questions about re-calibrating the FWD and REF power meter on the MFJ 1786
loop antenna control unit.  My FWD meter is poorly calibrated on the Hi range setting and I'd like to adjust it.

I've recalibrated similar MFJ meters (in their manual tuners) in the past. The
power calibration pots are clearly identified on the 1786 control unit PCB, so
the job should be easy, but I have two questions:

#1. Can I connect a 50 ohm dummy load to the control unit output without risking
damage? The loop manual warns in several places about attaching anything
in-line between the control unit and the loop because of the motor control
supply voltage that is carried on the coax.

#2 Normally, to calibrate these meters you balance the SWR bridge using a 50ohm
load, then you Tx into a dummy load and adjust FWD power pot to match the
power against a reference meter, then reverse the transmitter and dummy load and
set the REV pot in the same way.

But, is it safe to reverse the connections in this way on the 1786 control unit
to calibrate the REF power pot?

Any advice gratefully received,

73,
John
VK7JB

Hello,
The part you require is #50-0811-1 SA, PCB ASBLY, Parasitic, 811.

I recently installed it in my old 811.  Very easy install and replaces the old style wire inductor-over- resistor parasitic suppressors. It also upgrades  the plate leads with braided wire and the anode caps.  While I was at it, I replaced the Plate DC blocking cap, as you need to unsolder this from the original plate bus bar anyway when you install the parasitic PCB atop the plate choke.  The parasitic PCB assembly for the 811 and 811H are identical, except that there are only 3 units on the 811 board:  the 4th is unpopulated, as it's not needed in the 3 tube version, obviously.

Very easy upgrade and well worth the effort.

I've also replaced the tubes with 572b's. The old 811 runs like a dream now.

Regards,
John, VK7JB

Hello Group,
Renovations of my 18 year old 811 continue smoothly. Now I have a question about the power supply.  It's working fine, but the filter caps ( 4x 210uF @450V) are now 18 years old too.  Replacement caps don't look to hard to come by. I'd like to know if I should just go ahead and replace these ageing caps, or do I wait until they give up on me and deal with them then?  Looks simple enough to access and replace them:  they have screw terminals.  I note that in more recent editions of the AL 811, Ameritron have increased the caps to 270uF.  I don't imagine this change would have altered things much, but perhaps I'm wrong.

Any advice appreciated ( as usual )!
73,
John VK7JB

Dick,

Many thanks for your helpful advice. I've got hold of a suitable door knob cap and it'll be going in soon.
73,
John, VK7JB

Hi Roland,

I'm interested in this too, as I'm about to drop 3 new 572b tubes into my old AL 811.
I hadn't heard about this potential problem either.  Doesn't seem to accord with the empirical experience of others who've done the same.  There are many folks, in various discussion groups, who've report  postive experiences and long tube life after making the swap.  The designer of the amp, who contributes regularly to these discussions, has even endorsed the change over.

Let us know if you find any more info.

Regards,
John VK7JB

Hi Dick,

Thanks for your input on this too.
I'm wondering how I would mount a door knob cap in the AL 811?  The disc ceramic blocking cap just straddles the gap between the plate bus bar/parasitic PCB and the tank board.  I'm not sure how I'd manage the mechanics of it to make it secure.  Any tips.  And what rating doorknob cap would you recommend?

Thanks,
John, VK7JB

Yes, the price staggered me too, but if I wanted the goods, that was the only postage
option offered to me -  no negotation!
 
And what's more - to have 5 of the C13 disc ceramic caps shipped to me by USPS from
Ameritron, the shipping cost alone will be $27.00 USD.
 
I have had a lot of ham gear shipped from the US in the past, from ebay and other
merchants, and never have I payed this.  For example, not long ago, I had a great tuner
kit posted to me from a ham on-line - he charged $8 USD, and had a heavy  Palstar tuner
shipped in a HUGE packing box and that cost me $60 USD. Very reasonable costs.
 
Anyway, no use complaining - if I want the parts I have to pay the price, unless I can
source alternatives from merchants with less exploitative shipping costs. And that was
the reason for my original post,  not to complain about Ameritron.   I love my simple
old AL 811 and want to keep her firing along, so I guess I'll have to continue to pay.
 
Thanks for your interest.
 
John, VK7JB

Yes, I can order the part direct from Ameritron, but I've found out the hard way that ordering
from Ameritron is VERY expensive:

I recently bought from Ameritron direct, the parasitic suppressor PCB
for the 811, which cost $31.00 USD: a very reasonable price I thought.

BUT: the shipping to Australia cost me $99.00 USD!!! The whole package weighed only 9oz, or 260g, and measured 8x5x3 inches.

I questionned the amount by email and received no reply from Ameritron. I think Ameritron is clearly trying to disuade customers from overseas
from buying direct from them.
I'll be avoiding buying from Ameritron in future if there's any way I can source parts elsewhere. Ameritron mail order customers from down
under take note.

Thanks anyway for your advice.

Best wishes,

John

Hello again,

I'm looking to replace the DC filter cap, C13, in my 811.  The ratings for this cap are 1000pF @ 7.5kV.
I can source a 1000pF @ 6kV ceramic caps from a local supplier, but not the 7.5kV cap.
Can anyone tell me whether replacing the original cap with a de-rated one @ 6kV will make any difference in this circuit.  Is the 1.5kV difference signficant?

Many thanks,
John