CUSHCRAFT R7 Rescue and Refurbishment

CUSHCRAFT
R7
Rescue and Refurbishment
Jim Gerwitz
AC7FN
Summer - Fall
2011
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CUSHCRAFT R7
What it is...
Why you would want one....
Specifications.....
How they are built....
Issues that generally need repair....
WEB Resources...
2
What it is...
Vertical HF Antenna
7 Bands, 40 M thru 10 M
Requires NO Radials
Easily erected
Very small footprint
Easily found at swap meets
Usually cheap (approx. $50 to $125)
Usually defective
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R7 Specifications
Covers 10, 12, 15, 17, 20, 30 and 40 Meter bands
Has 3 dBi gain
Half wave on each band
VSWR 1.2:1 typical
2:1 SWR BW varies by band
Power rating of 1800 watts PEP
Typical radiation angle of 16 degrees
Height of about 22 feet
Wind load about 2.2 sq ft
Weight 12.3 lb
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They are easy to refurbish
They are fairly plentiful
Easy to erect and take down
Almost all of the bands are now active and open
Solar cycle 24 will be generally good for the next few years
Small footprint and easy to handle
Not too difficult to tune up, just time consuming
No band-switching required
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Construction
If you know how they are constructed, you will
understand how to repair them when we
discuss various problems
7 Bands means there are 6 traps
The 10 M trap is on the bottom of the stack, then the 12
M, then the 15 M and so on up to 30 M
40 M is determined by the overall length of the antenna
and has no trap
Not shown well here is the lower counterpoise of 7 each
49 inch radials
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Matching network
Isolator
Feed line
Connector
Balun
To antenna
Date of Mfg - July 83
Rivets
7
Matching Network and Typical Trap
MN-7 Matching
Network
30 M Trap
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TRAP DETAIL....
This 30 M trap has two
capacitors in series
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What can go wrong
Frayed fiberglass
Missing Pop rivets
Missing radials and clamp
Loose collar
Loose or missing
screws
Broken bracket
(When base is loose)
Open Choke
Broken cores
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Trap Issues
Poor or dirty connections at all clamping
points
Compressed tubes
under clamps
Corrosion under sleeving
Moisture in CAP tube
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11
Leaks
Corrosion
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Repair Process
First, the MN-7 Matching Network
6 turns white wire, 5 turns black
1) Completely remove PCB from box
2) Purchase new cores from MFJ and replace if/as needed
3) Reflow ALL solder on PCB backside, not just joints.
Make sure to reflow/re-solder all rivets!!!
(I overlaid track with braid and soldered everything)
4) Check continuity of bleeder coil
5) Insulate solder joints under core with tape
6) Reassemble with all Stainless steel hardware and new lock
washers
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BASE
1) If mast is loose in base, use
BONDO, JB Weld or other
epoxy to fill
2) Replace main mast screw if
missing, also large pop rivets
if loose. Use steel
replacements
3) Use Boat resin to seal
fiberglass matt. Then paint for
UV protection
4) Make sure all hardware is
Stainless steel and is TIGHT!
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TRAP REPAIR
• Remove all traps after carefully marking which end is up and which is first, second etc. If
your antenna comes to you partially assembled, be very suspicious that the traps are in
the right place, they may not be.
• Measure the resonant frequency and record it for each trap. Resources at the end of this
paper give you some measured values. I used a good old GDM and had good luck.
Resonance ‘dips’ will be weak prior to cleaning but very strong afterward. If you get
frequencies that are close to the band that the trap is for than you might possibly be able
to skip disassembly of the traps and inspection of them as in the steps below.
• CAREFULLY, disassemble each trap, ONE AT A TIME, and clean all clamping joints with
steel wool, Brillo pads or very fine emery cloth so that they are very clean. PUT EACH
CLAMP BACK EXACTLY AS YOU FIND IT! Do not mix parts from one trap to another.
Clean both clamping surfaces of each joint. Look for oval capacitor tubes distorted by
clamping. Put them back taking advantage of the out of round condition.
• I used a capacitance checker to measure the caps so that I could get them back exactly as
they were. The GDO works fine here.
• If you don’t have a Cap bridge to measure the caps, make mechanical measurements of
the positions of each cap rod, each cap dielectric and each cap tube so that you can put
them back as you found them. Check each trap with GDO or VNA for proper frequency on
re-assembly.
• Remove the HSS on each cap tube, slide them apart and examine them. Clean them with
alcohol and put them back together with new HSS sleeving but don’t shrink the sleeving
until tuning of the antenna is complete.
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TRAP Coils
• TRAP Coils are connected by the use of self-tapping screws that hold the coil end connection down against
the aluminum tube. Two types of screws were used: One was Stainless steel and one was zinc plated steel.
The earlier picture was the Zinc plated kind. You can check yours by using a strong magnet. SS parts are
only weekly magnetic. BOTH SUFFER CORROSION AND MUST BE CLEANED.
• While the capacitors are off of the traps, cut the HSS right around the screw location as shown in the prior
photo with the severe corrosion. Bend up the wire and use a Dremel tool with wire wheel to buff off all
corrosion on both the wire and the Aluminum tube, (both ends of the coil). While bent out of the way, take a
drill and drill the hole where the screw was, all the way through the trap. Do on both ends. Replace with a 1
1/2 in SS #6 screw and an aircraft self-locking SS nut from the hardware store. Bend the wire back down and
tighten the screws and nuts as tight as you can. Reseal the areas on each new screw and nut with
PERMATEX Black Adhesive Silicone Sealant.
• Replace the capacitors on the trap and reposition them exactly as they were in the beginning.
Do not shrink
the HSS yet, leave it loose.
• Measure the resonant frequency of the trap and make sure you are close to the original frequency.
I have
seen where this was suggested to be within 100 KC of the original but some I have done changed more than
that. Adjust them back close to your original reading.
• Use a plumbers round wire brush to clean the inside of the antenna aluminum tubing.
Every joint should be
bright and shiny at this time. Use steel wool, Brillo pads or emery cloth to burnish the tubing and clamps
where required.
• At this point you should have a solid base assembly with a rebuilt MN-7; 5 cleaned and retuned traps.
I
suggest you buy new clamps for the tubing sections.
• REMINDER: When reinstalling the cap tubes on a trap, put the new HSS on each tube but don’t shrink it.
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5 Traps??? I thought there were 6!!!!
This is a seven band antenna and that requires 6 traps. However the 10 M and
the 12 M traps are on the same trap assembly, so there are 6 traps but only 5 trap
assemblies.
This causes some issues in tuning since you can only adjust the length between
the antenna base and the bottom of the 10 M trap. To get 10 M and 12 tuned
properly the cap’s on one of the two traps will have to be adjusted. Just make a
note of this as we go on to final assembly and tuning.
When checking the tuning of the 10 and 12 meter traps, I suggest you short out
the one not being measured. There is interaction between the two tuned circuits.
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Assembly &Tune Up
•With a large tube of NO-OX or other aluminum anti-oxide material, assemble the
antenna exactly as per the manual. Make sure the dimensions are per the manual
and make sure you have the traps in the right sequence.
•Lubricate each tubing joint with NO-OX and tighten the clamps with a ratchet.
I
suggest you start with new SS hose clamps and put some oil on the adjustment
screws before use. You may adjust each one many times before you are through
and you will be glad you put grease or oil on them at the start. Pay attention to
which end of the traps go UP.
•Buy a 6 ft fence post at HD and put it in the antenna base with the round clamps
as per the assembly diagram. If you don’t have the round clamps you can buy
them from MFJ or you can just put bolts all the way through the fence post. EMT
or other pipe also works. Make it as close to the antenna base INSIDE diameter as
you can but it is not critical.
•In a clear area, away from trees, fences, or any obstructions, drive a second pipe
or small diameter fence post into the ground. The top of it should not be higher
than about 1 foot below the bottom of the antenna base.
•What I am going to show you is how I made a quick release clamp that allows you
to put up or take down your R7 in less than 5 seconds for tuning.
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QUICK MOUNTING CLAMP
•
•
•
The upper hose clamp holds the wire that goes
down to the lower hose clamp and keeps it
where it is always accessible. You could use
string or other things but the length has to allow
the lower hose clamp to slide over the pipe in the
ground when the antenna and it’s pipe are
standing next to it.
To raise the antenna, lift the antenna and mast,
place the base right next to the fixed pipe, pivot
the antenna up parallel to the fixed pipe. Lift the
lower hose clamp over the top of the fixed pipe,
then lower it and bingo!, the antenna is ready to
test for tuning.
Antenna
mast
Upper
clamp
Feed line
Wire
Lower
clamp
Laying it down is just the reverse.
Fixed Pipe
•
Very simple! You will do this several hundred
times so thank me now.
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TUNING......,
Are you ready???????
Before you start, make a chart of where you want the lowest SWR for each of
the seven bands. This is what you will use to tune each band.
Here are mine since I am not a CW person:
10 M 28.500 MHz
12 M 24.960 MHz
15 M 21.300 MHZ
17 M 18.083 MHZ
20 M 14.200 MHZ
30 M 10.125 MHZ
40 M 7.200 MHZ
CW folks may want lower resonant points.
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10 M
Since you have already set the correct distances for each band, our first task is to
start with the highest frequency band and see where it is tuned. Use your MFJ259/VNA/ Power meter or what ever and see where the lowest SWR is at. Write it
down and write down all adjustments each time you tune a band. If the resonant
frequency is too high, the length is too short. Too low and the length is too long.
This assumes the trap is tuned correctly and the capacitor is set in the right
position.
My experience on 10 M is that the resonant frequency is usually a bit high and you
will need to lengthen the antenna to get the resonant frequency down where you
want it. (125 inches vs. the manual of 123 in).
The bandwidth on 10 M usually covers the entire 10 M band (BW > 2 MHz) so it is
not a big issue. Typically I have been able to get the SWR below 1.3:1.
Next we will check 12 M. Here is where you may have issues.
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12 M
• Since the 12 M trap is on the same assembly as 10 M, you cannot adjust the antenna length and
make a change to the resonant frequency on 12 M without changing the 10 M adjustment that you
just made. You will have to adjust the 12 M capacitor. (You can do the reverse if you want. Set 12
M with the length and then change the 10 M capacitors for proper tuning).
• NOTE: INCREASING THE CAPACITANCE lowers THE FREQUENCY - That means sliding the rod
and/or the dielectric INTO the tube. INCREASING antenna lengths also LOWERS the frequency
• I found the dielectric position has a lot of effect on the frequency, so try to change the rod
position only to get the 12 M frequency where you want it. When you are successful, make
careful measurements of the rod and dielectric positions of the capacitor. Measure down to the
1/32 inch level.
• If you have lots of time, you can try changing both the rod and the dielectric position and
maximize the Q of the trap but I have had little luck in doing this.
• Do not shrink the HSS sleeving. Use blue painters tape to hold the dielectric and rod in the
correct position until completely through tuning all bands. Once all settings are complete and
you are happy with all adjustments, then you should shrink the sleeving and seal any places
needed.
• Once you have the 12 M frequency where you want it, make sure you have everything
documented.
• Typically I have been able to get the SWR under 1.40: 1. Not great but very usable with most
tuners.
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15 M
•
Measure the resonant frequency of the 15 M section. If it is off, adjust the length of only
the 15 M length to correct it. Do not adjust the dimension you set for the 10 M section.
•
It turns out that there were several versions of the R7 and some have different traps as
well as different length tubes so your manual may not be for the antenna you have,
unless you got it with the antenna. If you cannot shorten a section to achieve the
correct frequency, DO NOT CUT THE TUBING. Use the capacitor to tune the antenna.
You can play with the depth of the dielectric as well as the rod to adjust the Q of the
capacitor and subsequently the BW of the particular trap. This takes LOTS of work so
be prepared if you want optimum BW. I found little effect on BW.
•
Once you get it where you want it, record all data, use painters tape on the capacitor
and make sure all hardware is tight.
•
Recheck the tuning on 10 M, and 12 M to make sure it is where it should be. There will
be some effect but you just have to deal with it. Every time you adjust one band, check
the bands you already adjusted.
•
You will do this until you get it exactly like you want or until you get tired of doing it and
decide it is close enough.
•
I have been able to get the SWR down below 1.25:1 in most cases, sometime below
1.1:1.
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17 M and on ….
• Now check the 17 M frequency.
Adjust the 17 M length until you get the frequency
you want.
• Again, see the 15 M section for instructions and cautions.
Typical SWR is less
than 1.3:1 at minimum.
• Continue on and do each band until you have done 40 M.
Always recheck prior
bands and take data on dimensions etc. I always record the delta between the
factory setting and what it actually ends up to be.
• Make sure all joints get NO-ALOX and wipe off the excess or your hands will be a
mess.
• After a couple of bands you will get the hang of it and find what works best for
you.
• This requires patience.
You will find the quick release system described earlier to
be very valuable. I used Home Depot saw horses as the support to lay the
antenna on when assembling it and making all adjustments.
• Continue until all 7 bands are where you want them.
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Some typical tuning
curves
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40 M
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20 M
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17 M
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15 M
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12 M
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WEB Resources
•
WIKI--http://tinymicros.com/wiki/File:Cushcraft_R7.pdf
•
Manuals for 3 versions of R7
http://home.cogeco.ca/~ve3tes/r7manuals.html
•
Trap Resonant frequencies
http://www.iol.ie/~bravo/r7_vertical.htm
•
Another very good Refurbishment article
http://home.cogeco.ca/~ve3tes/
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