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

[Jim Wiley]

All other things being equal, 75-ohm cable has less loss per foot than 
50-ohm cable.  This assumes that the cables being compared have the same 
diameter, length, operating frequency, etc.  This is the primary reason 
that cable TV systems, telco inside-plant cabling, and other similar 
applications use 70-ohm cable.  Maximum power handling is obtained by 
using 30-ohm cable.  50-ohm cable represents the approximate mean 
between lowest loss and best power handling.


- Jim, KL7CC



On 10/26/2012 10:23 AM, 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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