[TowerTalk] Quad Plans

[NPAlex at aol.com]

To: Martin Sole _msole at loxinfo.co.th_ (mailto:msole at loxinfo.co.th) 
 
Martin,
I would like to make some comments on you Quad refurbishing plans - it may  
be 
easiest to address some of your questions within the text of your  message
using CAPITAL letters for distinction.
 
Norm W4QN
===============================
Subject: [TowerTalk] Quad questions,  long(ish)

It's that time of year again. With the better weather now here  and a 
little time to play I need to get outside and do some much needed  
antenna work. Way back during the stormy season my 2 el lightning bolt  
quad lost its 20m driven element, just snapped apart at a corner joint  
which if you ever tried that with aluminium welding wire you'll know is  
not an easy thing to do so I guess it was a weak spot brought on by  
several build ups and take downs. ====> I WOULD CONSIDER REPLACING
THE ALUMINUM WIRE WITH A STRANDED HARD DRAWN COPPER WIRE
ALUMINUM WORK HARDENS FROM FLEXING, AND IS NOT EASILY REPAIRED
AS A COPPER WIRE WOULD BE (SOLDERING).
We dropped the quad to a workable 
height on the tower and I currently  have a C3 on the top but it is 
evidently no substitute for the quad so I'm  keen to get it back up.

I have decided to make a few modifications and  would be interested in 
any suggestions before I get too far along with the  job.

First up I will be converting to separate feed for each of the 5  bands.
There will be a remote switch box located on the boom at the driven  
element end. ====> SEPARATE FEEDS IS MY PREFERRED WAY
OF FEEDING A MULTI-BAND QUAD.  I WOULD CONSIDER MOUNTING
REMOTE SWITCH AT THE MAST, AND USE 3/4 WAVE MATCHING
SECTIONS OF 75 OHM COAX.  
Feeders from the switch box will drop  vertically from the boom end to 
each element. At the feed point a balun,  T200-2 core with RG142, will 
interface each 
element to its feeder tail.  ====> ALTOUGH RG142 IS A GOOD CHOICE 
BECAUSE OF ITS SMALL SIZE, THE ADDITION OF 2:1 BALUNS FOR 
10M, 12M AND PROBABLY 15M NEGATES THAT ADVANTAGE.  THE 
OTHER PROBLEM IS THE PENDULUM EFFECT OF A RELATIVELY LARGE
MASS (THE BALUN) SWINGING ON THE ELEMENT WIRES - THIS WILL 
INCREASE THE STRAIN ON THOSE WIRES.   A LIGHT WEIGHT GAMMA
MATCH MAY BE A BETTER CHOICE.
Baluns will be housed in waterproofed  (really) ABS boxes.

A couple of questions:

The Lightning Bolt quad  has an 8' boom. From what I can determine this 
is quite wide spacing for 10  and 12 metres and performance might be 
improved if I were to re-locate one  of the elements, just for these 2 
bands, such that the spacing is closer to  5 feet. I can do that by 
adding a pair of cross arms at the different  spacing. Does that seem a 
likely improvement? ===> THIS WILL  OPTIMIZE THE TEN METER BAND
AND LIKELY TO ALLOW A 50 OHM MATCH.  DOING SO MAY ALLOW
ALL BANDS TO BE NEAR 50 OHMS.  I WOULD NOT USE BALUNS, BUT
CONNECT THE 50 OHM CABLE (UNBALANCED) DIRECTLY - THE
UNBALANCED FEED WILL NOT EFFECT THE PERFORMANCE. 
OR USING THE SMALL RG-142 CABLE YOU CAN EASILYFERRITE 
SLEEVES (ABOUT 16) AND CREATE A COMMON MODE RF CHOKE 
ISOLATING THE FEED LINE.

Cebik mentions quite emphatically that to  ensure decent performance from 
a 5 band quad, or any that involve multiple  bands, it is necessary to 
ensure that the driven elements of the un-fed  bands are shorted. In 
essence, when operating on 10m the 20m loop looks like  a 2 wavelength 
loop and will have significant currents in it unless it is  closed. It 
seems the only way to ensure this in all cases is by way of a  
switch/relay at the feed point. If this is a good idea I can arrange to  
do it quite simply. I have relays that should do the job, I am assuming  
that since they are open when the loop is fed and short when the loop is  
not fed these do not need to have great power handling. I have also  
thought about a similar relay switching method at the reflector to move  
the best F/B point. It probably sounds more complex in writing than I  
think it would turn out to be in practice. I will homebrew the switch  
box and provide separate contacts for the feed point shorting.  Thoughts?
=====> THE PROBLEM OF ACHIEVING THE DESIRED EFFECT 
IS COMPOUNDING BE THE EFFECT OF THE COAX CONNECTED 
TO THE UNUSED BANDS.  YOU HAVE TO TAKE INTO ACCOUNT 
THE LENGTH OF THE ATTACHED FEED TO EACH BAND, HOW IT
IS TERMINATED (ANT SWITCH, OPEN OR SHORTED) AND THEN
TRANSLATE THAT TO THE FEED POINT.  TO ACHIEVE THE "SHORTED"
EFFECT ON 20M BAND, OPERATING ON 10M, THE 20M FEED LINE SHOULD
BE NEAR 1/2 WAVE AT 28MHZ, IF NOT AND IT IS NEAR A ODD 1/4 WAVE 
IT COULD REFLECT AN "OPEN"   -  SOLVING ALL THE  VARIATIONS
IS COMPLEX, AND IN THE FINAL RESULT I AM NOT SURE IT MEANS
A LOT.

I had thought of dropping a catenary rope from the boom end  to support 
the feed points but now think the the combination of several  feeders and 
any associated switch wiring should be sufficient support in  itself. If 
I really should need some additional support what would you  recommend? 
Dacron, phylistran and other wonder products are unobtanium  here.
====> SEE MY COMMENTS ON THE WEIGHT AND PENDULUM EFFECT
OF BALUNS.  STRIVE FOR 50 OHM SOURCE Z, AND CONNECT THE 
RG-142 DIRECTLY, NO BALUN OR USE FERRITE SLEEVES TO 
ISOLATE THE FEED LINE. 

Where do you buy T200-2 cores? I know I need  to order these, just 
looking for a simple easy supply outlet.

FERRITE SLEEVES ARE AVAILABLE FROM "THE WIREMAN" 
IN A KIT OF 16 I THINK.

That's enough for now. Any ideas much  appreciated.

Martin, HS0ZED


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