[Elecraft] LDG antenna tuner with 132 foot balanced dipole similar to a G5RV antenna

Fri Oct 26 15:17:45 EDT 2012

The significance of the 75 Ohm line is a matching section that 
introduces a reciprocal impedance that brings the SWR to 1:1, or nearly 
so. However, this approach does not address the balanced to unbalanced 
issue. And, this is generally a one band match. A 1:1 choke type balun 
will still be needed.

The approach may be technically correct, but it is more work than I feel 
it it is worth. I still suggest the simpler approach I noted earlier as 
it does work and requires zero engineering knowledge. You just hook up 
the 450 Ohm ladder line to the antenna on one end and the other goes to 
the 4:1 current balun. Low loss coax is connected between the tuner and 
the balun. The operator just tunes the affair until he gets to an 
acceptable SWR. Nothing else is required, no fiddling around, just 
operation. 100 feet of 450 line and a few feet of coax will yield losses 
in the  1-1.5 db range with SWRs up to about around 10:1. This just 
isn't worth all of the fiddling, IMHO. I try to always adhere to the 
KISS principle.

73,
Barry
K3NDM

On 10/26/2012 2:23 PM, Bob Stephens wrote:
> What is the significance of using RG11 75 ohm coax vs 50 ohm?  Sorry if this is a dumb question.
>
> Bob AF9W
>
>
> On Oct 26, 2012, at 10:39 AM, N5GE wrote:
>
>> All,
>>
>> There's a trick I learned from an antenna engineer acquaintance
>> recently for tuning 450 ohm feedline's with a 1:1 balun with 50 ohm
>> coax feeding the TX.
>>
>> Here it is:
>>
>> 1. Assemble three RG11 coax lengths.  One coax 6 feet 7 inches long,
>> one coax 6 feet long and one coax 9 feet long.  From these three coax
>> lengths you can test with four different lengths of 75 ohm coax;
>> 6' 7", 12' 7", 15' 7" and 21' 7".
>>
>> 2. Beginning with the 6' 7" cable and connect the coax between the
>> balun and the 50 ohm feed line, repeat the test below with each of
>> the lengths above.  Connect the cables with double female connectors.
>>
>> 3. If you have an analyzer (MFJ analyzers work fine for this) that
>> gives the Rs, Xs and SWR go to step 4; If you don't go to step 6.  If
>> you have an AIM 4170 analyzer, you are in luck.  The AIM allows you
>> to do single shot analysis of a single frequency, displaying more
>> information than you need for this test.
>>
>> 4. Record the Rs, Xs and SWR of each combination at the frequency you
>> are interested in.  A spread sheet makes this a little less tedious.
>>
>> 5. After recording the calculate the SWR the tuner will see with the
>> following rules:
>>
>> Disregard Xs.
>>
>> If the Rs is greater than 50 divide the Rs by 50.
>> if the Rs is less than 50 divide 50 by the Rs.
>>
>> The result will be the SWR the tuner is working against when
>> searching for a match.
>>
>> In my case I am looking for a calculated SWR lower than 10:1 to help
>> my KAT500 tuner find an acceptable match to my KPA500 and NVIS Loop,
>> which works on all amateur bands for close in Non-DX rag chew
>> communications.
>>
>>
>> 6. You can still do the test, but you will need to do it by testing
>> with the tuner for the best combination of matches at each frequency
>> of interest.
>>
>> If you are going to do the tests, remember to use the MINIMUM power
>> your tuner can tune with and allow time for your rig to cool down
>> between tests.
>>
>> 7.  When you have completed your tests measure the length of the
>> joined test coaxes, including the double female connectors, and make
>> a piece of RG11 that length.
>>
>> DISCLAMER:  I'm not a real technical guy and probably don't know as
>> much antenna theory as many of you, but what I have described worked
>> for me.  Your experience may not be the same.
>>
>> Amateur Radio Operator N5GE
>> ARRL Lifetime Member
>> QCWA Lifetime Member
>>
>>
>>
>> On Fri, 26 Oct 2012 05:42:20 -0700, Ron Midwin
>> <ronmidwin.mo at gmail.com> wrote:
>>
>>> I recently bought an LDG AT600 Pro Automatic Tuner.
>>>
>>> I seem to be able to get a low VSWR on 80 & 40 meters, but on 20, 15, & 10
>>> meters the best it will do is ~ 1.8:1.
>>>
>>> My "G5RV" (It's really a Van Gordon 80 meter all bander fed with 450 ladder
>>> line, 33 feet long, and then transitions to coax thru a 1:1 choke balun;
>>> coax is ~ 6 feet long.
>>>
>>> Has anyone found a configuration that works better on all the bands?
>>>
>>> Previously I have been using an MFJ 962D manual tuner that is able to tune
>>> down below 1.2:1 on all bands.
>>>
>>> AE6RH
>>> K3 S/N 1997
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