Examining the Johnson Matchbox ATU

<< < (3/7) > >>

ke7trp:
Other then driving the cost of the tuner up, does this extra two sections have any ill effects?  I cant see any.  I own two KW matchboxes. I use one everyday.  I own 3 of the 275 watt versions.  I dont use them much anymore. I have heard for years the KW has better RX.  I cant imagine that being True.  But I have heard or read somewhere, that the large metal box makes a difference in the performance.  Something about the Q.  I am not an expert on this subject, I am just trying to remember what I read or heard over the air. 

Maybe I will Drag a 275 in, lay it ontop of the KW, And Test this out on WWV with the IF scope. Maybe I can determin if there is any truth to the statement.  I have a feeling that one person wrote this somewhere and hams picked up on it as fact. 

There was also discussion about having the RX bypass the KW matchbox.  EFJ gave provisions for that if you used the built in TR switch.  Some people had much better results NOT using the tuner on the RX.  This of course turned out to be some RXers being at 300 ohms input, Military at 98 ohms and some being at 50 ohms.  Depending on the atenna and the RXer, You might have been better off not going through the KW. 

A simple test is to tune the tuner flat for transmit.  Monitor a signal and retune for highest RX.  I was very suprised to see some of my RXers go up a good deal on Recieve when tuning OFF the Transmitter settings.  Some have an antenna TRIM or TUNE knob and I could bring this back with that setting. Some do not have that feature.  I ended up trying to make a matching balun for 50 to 98 ohms but gave up and ran out of time.

C

K5UJ:
This is vy interesting.  Maybe EFJ used the four section cap to make folks think they couldn't homebrew their own tuner? hi hi.

That four section cap is hard to find isn't it.   Thanks Walt for this information.  I know the KW MB has faults but it is still by far the best manufactured bal. tuner I have ever used.  Didn't know about the inductor heating up though.  At least I have not seen any vswr creep during long transmissions like I saw with my old bal. tuner, the Bliss Matchmaster. 

You all who have tried the var. cap in series with the link, pse tell what v. and value you used.

It'd be interesting to see a side by side A/B with the 275 w. and KW; guess all you need besides the two tuners is a big knife dpdt switch and a coax switch.   

On mine, there's no rx relay, that all got taken out by some past owner.  I'm gg to go see if there is some higher noise level settting on rx other than what I have it set on for 1:1 tx vswr.

Well I went downstairs to the shack to power everything down for the night and found out in my case on 3558 KHz the low vswr setting was also the high rx noise (i.e. sensitivity) setting.

Rob

WA4JK:
Good info and discussion folks. I'm going to poop the case on my Murch 200b to see if I have a two or four section cap in it. I T/R the tuner out of the receive circuit.

WA1GFZ:
I think the Q of the match box stripped off some RX noise. Today most receivers have a broadband input unlike the days of tuned preselectors.

Steve - WB3HUZ:
The Q of the Matchbox is unlikely to be high enough to have a bandwidth more narrow than the IF bandwidth of any receiver. So, unless the front-end of the RX is overloading, there will be no SNR improvement from the Matchbox.

Navigation

[0] Message Index

[#] Next page

[*] Previous page