[TowerTalk] (no subject)

[K7GCO@aol.com]

In a message dated 9/16/00 10:16:31 PM Pacific Daylight Time, kq2m@mags.net 
writes<< 
 Gentlemen,
  Can anyone tell me what  Heathkit 1250 & Millen 90651 grid dip meters with
 coils are worth?
 
 I am thinking of buying one to help me with making 1/4 wave stubs.  Are
 either or both of them adequate for this job?
 
 Tnx for your help.   73 >>

The type of coil has nothing to do with accuracy.  Some of the cheaper GD's 
have poor connections on the coil pins.  Some GD meters are easier to work 
with and their calibrations vary.  Since there is no buffer between the 
oscillator and the circuit sucking up the RF, the amount of RF loading to the 
tank circuit pulls the frequency and changes the current in the grid circuit 
hence the name "Grid Dip Meter."  Even if perfectly calibrated it is altered 
by the amount of loading to the test circuit.  This is in direct proportion 
to the depth of the GD meter dip.  The closer the coupling and the deeper the 
dip the more the calibration is altered.  The lighter the dip the less the 
calibration is altered.  At the bottom of a light or deep dip the frequency 
is essentially the same.  The exception is if the GD coil is so close to the 
tuned circuit it affects it's resonant frequency some in a high impedance 
area.  Don't worry about it when grid dipping a shorted 1/4 WL end of a 
transmission line and the shorted end is the only end that will absorb RF 
from the GD coil.  Being too close to a tank circuit could upset the resonant 
frequency.  Hi Q circuits will allow the greatest separation distance and 
Lo-Q circuits will require the closest.

With the procedure to be described you will be able to make the most accurate 
resonant frequency possible with the simplest of test instruments.  I've 
cross checked it time after time.  It's the most used test instrument in my 
shack and a GD meter should be issued with every ham license paid for by the 
Ham Industry.  It will enhance many technical aspects of your ham radio not 
possible any other way.  The other piece of equipment that I consider an 
absolute must for the mechanically inclined is a drill and tap set, a hand 
drill or a Drill Press if possible and a complete set of standard, # and 
metric drills.  If you plan to and can do mechanical things, you will come 
around to my way of thinking or you will have a lot of equipment that is not 
made very well and "constant maintenance."  

Assume you want to GD a 1/4 WL line (shorted just on one end--GD there) at 
3.562 MHz.  1/2 WL lines have to be shorted on both ends.  Cut it according 
to the resonant frequency formulas, the VF of the line and about 1% longer 
(246/MHz X VF = ft for 1/4 WL).  Most solid dielectric coax has a .66 VF but 
pollyfoam is higher and varies all over the place even on the same roll. 

Open wire line is easy to short out.  Clean the tips and twist for say 3/8".  
Coax is another problem with a simple solution.  The least error of 
measurement is made with the least additional wire added to make a loop large 
enough so as to encourage RF from the GD coil field to jump ship and couple 
in the tuned circuit.  This is not a problem with open wire line as the end 
is a loop already.  

Install coax connector on one end.  Make up what I call the "K7GCO--1.) 
Short, 2.) Short-Short & 3.) Short-Short-Short."  This will make it possible 
to GD from 160M-UHF.  The idea is to use the "Shortest Short" possible for 
enough coupling to get a "perceptible dip."  Here is the way they are made.  
The #1 Short is a SO-239 female receptacle with a loop of wire just long 
enough to go around the GD coils and soldered to the center terminal and the 
receptacle frame.  This loop size is what is needed for adequate GD'ing from 
160M to about 40M.  The #2 Short-Short has a soldered wire loop about 1/2 the 
diameter of the GD coil for 40M-15M.  The #3 Short-Short-Short is the 
shortest wire short you can solder between the center terminal and 
receptacle.  This is all that is needed for 10M and up.  This "short wire 
length" and the additional length of the SO-239 connector (about 3/8") only 
starts to add additional length of any consequence say at 2M and up.  Do some 
math and figure out what the equivalent extra length of say 1/2" would lower 
the resonant frequency and GD it there.  Trust me it works.  You won't have 
to worry about this additional length error on 10M and lower.  

Assume you are GDing a 1/4 wave of coax.  Add the appropriate "GDing Short" 
to the coax connector.  The coax can also lay on the right on the ground.  
Open wire line has to be supported off the ground about 5'.  1/2 wave open 
wire needs to be shorted on both ends and rope can be used to tie it to the 
proper supports about 5' off the ground.  The open ends of a 1/4 WL of open 
wire line stub cannot be attached to directly as the Hi-Z at the open end 
wire tips would be loaded and the resonant frequency will be changed.  Attach 
a cord to the center of a spacer.

Set receiver at desired frequency with a stub of wire stuck in the coax 
switch to hear the GD signal which is not strong.  Sweep the GD meter across 
this range holding the GD coil near the coax Short.  If close enough the GD 
meter will dip indicating RF was absorbed by the shorted feedline.  If the 
degree of coupling is just light enough to obtain a perceptible dip, the 
frequency reading on the dial is accurate will be the least affected.  If you 
had cut the length 1% longer than necessary it will GD slightly lower than 
the desired frequency.  Start trimming the other end until you get close to 
the desired resonant frequency.  I often hold the shorted end of the coax in 
a vice and set the GD on the table so both are stable in order to maintain a 
stable GD frequency.  Any change of loading will alter the frequency.

Here is a guide line to quick determination of the GD frequency.  Plan A is 
to just keep tuning the receiver and finding the GD signal when lightly 
dipped.  Plan B is to leave the receiver set at the desired frequency.  As 
you sweep the GD through the desired range you will see and hear a "dip-blip" 
or hear and see a " blip-dip".  The dip is the meter dip and the blip is the 
sound from the receiver as you sweep the GD past the receiver frequency.  If 
you can't hear a blip, add a longer wire to the coax switch and bring it near 
the GD meter.  If you sweep the GD up from the below the desired frequency, 
the sequence of the "dip-blip" or "blip-dip" tells you whether the feedlines 
resonant frequency is either lower than or higher than the receiver 
frequency.  You may have to think about that a bit.  This is a simplistic 
description but "it fits."  With practice you will get the hang of it.  The 
idea is to get the "dip & blip" or the "blip & dip" on the "same frequency."  
 Be careful and walk up to it with progressive smaller cuts.  If you 
accidentally cut off too much it's a "cruel cut" comparable to another cut 
and that can't be replaced.  You can start over with another coax.

Practice on some old coax first.  I had an article on How To Grid Dip Coax in 
CQ some 30-40 years ago.  I've GD'ed a 1000 or 2 of feedlines and these 
procedures will enhance your measurement accuracy and speed.  If the truth 
were know and I'm here to tell you what it is--you cannot live without a GD 
meter, without knowledge of how to use it and expect fulfill the technical 
requirements of a completive Ham Radio Station.  That's the "K7GCO Grid Dip 
Meter Law 101."  Get a GD meter, learn to use it and enjoy antenna progress.  
I like the old tube Millen GD meters except on 2M.  It's a bit tough to get a 
dip. The old Booton GD meter works great on 2M and above.  

I did find a concept you should know.  You can parallel 2 1/4 WL stubs to 
obtain a stub of half the Zo or any Zo using ohms law.  But I found that when 
I paralleled 2 stubs of the same resonant frequency, it lowered about 5% and 
I had to trim both for the desired resonant frequency.  Also on 10M and up 
you can make a very simple short by making a hair pin with about 1/8" spacing 
and stick it in the end of the coax and grid dip it.  Virtually no GD error 
whatsoever.  

I have the GD attachment for the MFJ but haven't used it yet.  Potentially 
this could be the best of all instruments with it's direct frequency reading. 
 Has anyone really checked this out yet?   K7GCO




--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com