[SOC] Matching G5RV

Mon, 18 Aug 2003 14:12:00 -0400

At 8/18/2003 05:51 PM +0100, Paul Bartlett wrote:

Hi All,

I have a conundrum to solve.

Hmmm.... Trojan or Ramses?

I have a K2, a full size G5RV
(104') and an ex-miltary ATU (Redifon type

A.C.U.9) kindly loaned to me by Paul MW3PWA.

The ladder-line phasing section of the G5RV terminates in a 50R
co-ax

connector and is immediately followed by a 10 turn co-ax 'choke
balun'

(about 6{quot} in diameter). There's then a 30' run of co-ax to the
shack.

Personally .... I don't see a need for the choke balun, but tis my
personal

opinion ..... and opinions are like noses ..... everyone has one ....
some smell

..... some don't.

The ATU takes an RF input via a BNC
connector which runs to the K2.

Disassembling the ATU I see that the hot input passes to a bank of

switchable capacitors down to  ground and thence to variable series
inductor

(rather nicely made).

There are three output terminals: Earth; the other end of the inductor;
and

a third terminal connected to the second via a high-voltage cap which
value

I can't determine.

If I understand the description ....

input 
----------|--------|--------|--------UUUUUUUUUUUUU--------|--------
output

|       
|       
|                                            
|

---      ---     
---                                         
---

---      ---     
---                                         
---

|       
|       
|                                            
|

|
-------|--------|---------------------------------------------|---------
earth ground

|  

Ground

Looks like a pi network .... The input caps are switchable in or out ....
the output cap is switchable in or out 

Looks like a pi network ....   If the only {quot}tuning that
can be done is the switchable input capacitors, I'd think it may be
usable.  If one of the input caps and the output cap are variable,
I'd think it would be real usable.

Questions:

Is the ATU sensibly usable at all with my mix of antenna and feed? If
so,

how?

Here's what I would do ....

Get a cup of tea.

Hook the input up to the K2.  
Hook the output to the coax feeding the G5RV.  
Sit back, drink some tea, look at the set up and confirm that it is
correct.

Listen on, let's say, 20M .... CW of course!

Vary the inductor until you get maximum noise or signal.

Switch the capacitors in and out until you get maximum noise.

Vary the inductor some more for maximum noise or signal.

Write down, while sipping on tea, which capacitors are switched in, where
the setting on the inductor is, and if the output capacitor is in or
out.  And the S meter reading.
At a watt or two, touch the tune button, or key the K2 with a straight
key and check the SWR.

If it's not below 1.5 or so, vary the inductor to see if it goes up or
down .... adjust accordingly.

Get another tea.

Check if switching around with the input caps lowers the SWR.

I think you see where this is going ..... 

Do I need a further balun after the
ATU to match the unbalanced feed?

No .... but then I'm not a fan of baluns.

Should I ground the ATU via it's
EARTH terminal?

Absolutely, unless it works better without a ground!

73    hank    k8dd
soc #69

Any and all suggestions
welcomed.

Best,

Paul M3CRQ :-)

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*/     Hank Kohl 
K8DD      [email protected]
*/     ARRL
TS             
http://www.qsl.net/k8dd

*/     MI-QRP - Vice Pres.  QRP-ARCI -
Director

*/

If God intended you to be on single sideband, he would have given you
only one nostril.

- Steve, K2PTS (SK)

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