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[TowerTalk] Distance from exciter, amp, tuner, dummy load, antenna

To: Tower Talk <>
Subject: [TowerTalk] Distance from exciter, amp, tuner, dummy load, antenna
From: "Roger (K8RI)" <>
Date: Mon, 09 Aug 2010 00:47:48 -0400
List-post: <">>
Not sure which reflector this one should go to as it includes coax 
length, antenna tuner, dummy load, and antenna.
I'm going to try TT and AMPS, but not cross posted. I hate cross posting 
to other reflectors with a passion, particulaly when I'm not subscribed 
to the other one with the same e-mail address.

At-any-rate, when I tune up the amp or match the exciter to an antenna, 
I have a coax switch between the exciter and amp on relatively short 
sections of RG8-X.  The allows me to switch between the exciter and the 
antenna analyzer. Between the amp and tuner is a minimum length of 
LMR400UF as well as between the AUX connector and DL5K.   So the thought 
is I can tune the antenna with the Palstar AT5K, then switch and tune 
the amp into the DL5K dummy load by going to the aux position which is a 
straight through to the dummy load. Of course there are lead lengths 
inside the AT5K and it's not a small tuner. Tuner to DL is only about 
8".  It's about a foot from the amp to the tuner, and from the exciter 
to amp (through that switch) is about 2'.   The idea is to go on the air 
without ever tuning on the air.  This has worked very well on the lower 
bands up through about 20, but I ran into two problems. The first is the 
R and X vary from 50 to about 30 ohms and the X goes from 0 to over 60 
ohms.  It appears this is a function of the total distance between the 
exciter and the dummy load through the antenna tuner including all coax 
which above calculates out to about a minimum of 44" plus the distance 
inside the tuner. It *appears* I can ignore the distance inside the DL5K 
as it's coax that is compensated and does not appear to enter into the 
equation with the analyzer hooked directly to the input.   Hooking the 
antenna analyzer to the dummy load confirms this with a virtually flat 
SWR of 1.1:1 from 1.8 through 30 MHz with R remaining 47 ohms across the 
range. Not quite perfect as I was aiming for 1.0:1, but it'll do.  With 
the normal equipment configuration and leads as short as possible, the 
best I can get at 30 MHz is a tad over 3:1.   I don't normally use the 
amp on 10, but still, I'd like the option without having to tune up on 
the air. I can match the coax lengths so the distance from the analyzer 
is the same as the exciter.

I did have one resistor in the DL5K apparently change value a bit which 
was attributed to overheating, but as the most I can put into the DL5K 
is 2KW when pushing for tilt and then a slight back off of the drive, 
that puts me very close to the continuous ratings of the DL5K which BTW 
is a fan cooled, dry load.
So as I try to keep the key down time limited, it's close to the 
continuous ratings with the max I can get out being 2KW or a bit less 
than a single 8877, I doubt it was overheated. Cooling is via a pair of 
fans that don't have quite enough power to move the DL5K off the desk 
when in high gear<:-)) They move a LOT of air!

So, I'm trying to figure out how to get the DL5K to represent R=52 and 
X=0 across the HF bands and even 6 meters without having to hunt for 
specific coax lengths for everything from 20 on up through six.

Any thoughts and suggestions?  I could tune up on the air like most do, 
but I'd prefer to avoid that and can at present.


Roger (K8RI)

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