Sunday, July 24, 2011

Simple antennas for the Hawaii ham operator, part 4

This weekend has turned into a decent antenna day for amateur radio operators on Hawaii Island.  Since I completed most of my newsroom duties early this weekend, I was able to work on a few antenna ideas I first tried in my early days as a novice operator.  I pulled out my antenna notebook for 1978 and found a bunch of antenna ideas under the November category--a fairly wet month according to historical records.  That may have been the reason I fashioned a few "quick and dirty" verticals capable of being erected and taken down between drenching tropical showers. 

One of my vertical helix antennas proved useful and fairly cheap to construct.  Borrowing freely from the "ARRL Antenna Book" and various publications from the RSGB (Radio Society of Great Britain), I assembled a compact portable unit that could be used in an emergency.  I decided to re-build this skyhook on Saturday.  It works well, considering its narrow bandwidth sosme high angle radiation. 

I found an unused 10-foot piece of schedule 40 pvc pipe under the qth and decided to press this plumbing project left over into antenna service.  I spiral wound 66-feet of #22 gauge hook-up wire on the pvc pipe, attached a 9-inch diameter aluminum pie plate to the top for some top loading,  hooked up eight, 10-foot radials with detachable clips, and fed this contraption with some 450-ohm twin.  The twin lead went to my DX Engineering 4:1 balun and then to the trusty Drake MN-4 ATU.  A small piece of RG-58 coax mated the system to the venerable Swan 100-MX.  Does it work?  Yes, it does, with a very narrow bandwidth.  The helix can also work  the amateur bands from 40 to 10 meters, given that tuning will be very critical.  According to the "ARRL Antenna Book", a spiral wound helix of approximately 1/2 wavelength for the band of your choice should give you a compromise 1/4 wavelength vertical.  The impedance measured at the base of my homebrewed vertical helix was around 5 ohms, so there are quite a bit of losses.  The losses dropped once I added several more radials.  There is a difference of opinion on the length of the radials--some say the radials should be at least a 1/4 wavelenth for your design frequency,  Others I have read claim the radials should be as long as the length of the antenna support--in my case, around 10-feet.  Since my backyard is small, I will use as many radials as I can.  The helix does work and it could help you get a signal launched if there is no other choice.

I've modified the original support, dividing the 10-foot pvc pipe into two, 5-foot pieces, which can be joined to set up the antenna.  The antenna, 10 radials, a small antenna tuner, and a deep cycle battery now reside in the back of my Honday Odyssey van, ready for use.  In the future, I will set up this emergency antenna at a public park and see what I can far, the old Yaesu FT-7 (at 10 watts) has garnered a few contacts using this system at the home station.  I do better with the Swan 100 MX, but for portable use, I'll stick to the old Yaesu.

Next on the design table is the building of a vertical dipole designed around the helix idea, with each section using 66-feet of #22 gauge wire, fed by 450-ohm twin lead.  Antennas along these lines can be found in most antenna books.  I tried this arrangement a few years ago, with some success on 40-meters.  Of course, results will depend on  your location, propagation, and available materials.  My "junk box" had a good source of materials, so I didn't have to spend much for wire and pvc pipe.

Another approach for those of us challenged by CC&Rs, noisy neighbors, and lack of space would be to build a simple dipole using "ham stick" mobile antennas for your band of choice.  I've used this type of antenna in the past.  This antenna will get you contacts, although not as many as a dipole up 40 to 50-feet.  Be creative with what you have--the idea is to experiment and to get on the air despite the limitations of your surroundings.  Our amateur radio experience is a continuous learning process.  For me, the stress relief and brief escape from our severely misaligned world are worth the effort.  There's something intriguing about launching rf into the "ether" and seeing where it goes.

Have a good weekend.....Aloha es 73 de KH6JRM.