Building a WXSAT QFH antenna
I followed the QFH design guide by Bill Sykes - G2HCG and Bob Cobey - G0HPO and heres how I got on.
Thursday, 25 July 2002|
In WXSAT circles, the QFH is regarded as one of the best omni-directional circurarly polarised antennas available. It has a good radiation pattern, and provides a reasonable null at the horizon, which prevents Wide Area radio-paging from interfering with the weak WXSAT signals.
Firstly, read the QFH design guide at http://website.lineone.net/~askrlc/
I decided to build another QFH, mainly because I was a little dissapointed at the tall QFH I've been using for a few years. I seemed to be getting nulls in the signals when satellites were overhead which was a little annoying. After a good search round the net, I foind the above article, and decided to give it a go as it's a fairly quick project.
A trip to B&Q with the other 1/2 to look at garden furniture resulted in the purchase of all the components to build the QFH, and only one small table for the garden! A result.
As stated in the components listing on Bill and Bob's pages, I purchased:
10 meters of 8 mm Copper Tube
10 of 8 mm 90 Deg. End Fed Elbows
1 length of 32 mm PVC waste pipe
A pack of small selftappers
After grubbing around in the garage, I managed to locate a suitable length of UR43 or RG58 Coaxial cable for the Balun and feed. I also used a piece of Nylon bar about 1" long that was a tight fit into the 32mm waste pipe - more on this later.
Firstly, following the design, I drilled the 32mm waste pipe with four holes at the top, ninety degrees to each other. The wastepipe is cut to length at 1000mm. The top four holes will hold the top four parts of the antenna. Next, the bottom holes, in the same plane at the four top holes. The bottom holes hold the long and short parts of the elements. I marked the two bottom element sections in the middle, and then 17.5mm either side of the middle. This helps when centreing then in the wastepipe. Once the bottom elements are in the middle, use cable ties to secure them to prevent lateral movement.
The 8mm microbore tubing was cut to length and the ends prepared. I used a marker pen to write the lengths on each piece after sawing to avoid confusion in the heat!
Bending the elements is somewhat of an art. In the article no advice is offered on bending the tubes as its not very easy to describe. The method I used is as follows:
Firstly, get all four pieces that have to be bent into partial helicals and cable tie together at three inch intervals. Use a pair of pliers to pull the cable ties very tight though. Next, find something round thats around 300mm diameter and maybe two feet long. Use some old carpet or cardboard to wrap round the item to increase the diameter if need be. Next, after making a note of bending direction, grab the bunch of elements and carefully wrap round the cardboard / carpet. once this is done, hold the ends up to the holes in the 32mm wastepipe and if necessary, pull length ways to make them fit.
Next, following the article, solder the elbows and pipework. Use some masking tape to hold the antenna together though as it's got an annoying habit of springing apart. Wet rag must be used on each bit of pipe to prevent the plastic waste pipe getting too hot and melting.
The connections at the top must be made next. Bob and Bill suggest making a small PCB, but since Carl - G6NLC has my PCB stuff at the moment, I decided to apply a large helping of bodge-a-tronics. The Nylon plug I mentioned earlier was prepared as follows: Firstly, a hole around 8mm was drilled down the middle for the coax to pass through. Next four holes of 2mm are drilled at ninety degrees from each other, 4mm in from the outer diameter. The four top elements have 3mm holes drilled 4mm from the ends and the self tapper screws are put through these holes and screwed into the nylon. I used a piece of Nylon about one inch long.
Holes are drilled in the wastepipe for exit and entry of the coax for the choke coil. I decided to feed the coax back into the centre of the pipe so it can exit from the bottom. I drilled several pairs of holes down the entire length of the pipe and cable tied the coax inside by passing the tie into one hole, round the coax and out of the other hole.
Once the four top elements are in place, I used a large soldering iron, around 75Watts to solder the coax inner and braid to the relevant element sets. In this case, the outer is connected to a long and short element pair, whilst the inner is connected to the remaining long and short element.
The top of the pipe was filled with hot glue to provide a decant waterproofing, and an end cap fitted. The entire length of waste pipe is filled with expanding foam for water protection and to prevent spider ingress.
Since I still had the old QFH, I examined it for faults, and found one of the solder joint to be dry. This has now been fixed, but of course left me with the problem of having two antennas to connect to the RX2. I solved this buy knocking up an antenna relay that is controlled by a voltage up the coax. No volts means the new QFH is connected, whilst volts connects the tall QFH. A capacitor prevents DC being dead-shorted by the QFH antenna's. Inside the RX2, provision is made already for feeding a voltage up the coax for a preamplifier or a downconverter. I've switched this and added an LED indicator so I know which antenna is selected.
Below are a few photos I've taken of the completed antenna.
Bill and Bobs QFH [Click here]Paul Hayes QFH design [Click here]David Taylors QFH [Click here]WX-TO-IMG wxsat decoder software [Click here]Tall Narrow QFH antenna design [Click here]137Mhz QFH antenna [Click here]PA0ROJ QFH Antenna [Click here]
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