m0zsa
New Member
Greetings all. 
If I told you I made an slinky antenna 650mm in diameter and I can transmit 80 to 17m with 100 watts with a SWR of less than 1.2:1, I think you would tell me I am smoking my socks.
In Practical Wireless, November 2003 John Heys, G3BDQ, wrote about a Slinky Hula loop antenna he made with a slinky, a hula hoop and a 120pF capacitor. So using his loop antenna design and a bit of my mechanical skills I made the following version.
It comprises of 32 turns of an original Slinky, 2042 mm of 1/2 plastic water pipe, plywood base, strong string, and the most important one 500pF variable capacitor.
I just happened to have a Jennings vacuum variable capacitor, obtained from an Amateur Radio junk sale in South Africa. This is the Rolls of capacitors, not for the faint hearted when buying a new one so if you are going to get one try and get it the same way I got mine (as cheap as possible!). I'm told even if it has lost its vacuum you can still transmit up to 100 watts(?)
If you are going to experiment with standard variable capacitors I would suggest a slow-motion drive. I found a 120pF did not tune down as low or as high as the 500pF and was very sensitive to the touch.
I started off with the pipe, marked the centre and then marked 29mm either side of the centre and then 58mm 15 times either side of the centre. I then drilled 5mm holes right through the pipe in a straight line.
The Slinky is available from many sources, mine from Maplin in Brighton (UK), though many toy stores also stock these items.
Count 32 rings of the Slinky, cut off with a strong pair of wire snips.
Screw one side of the pipe to a suitable wood base making sure it is in the same plane as the rest of the holes.
Thread the Slinky over the pipe, bend the pipe round and then screw the other side of the pipe to the wood base.
Start with tying one full ring of the Slinky to the pipe and then one on each hole ending with a full ring again (See the sketch for the circuit diagram and tie down info).
Ken G3WYN supplied a suitable knob for the capacitor as the 6mm shaft was heavy on the fingers!
The antenna tunes anywhere between 80MHz and 17MHz with a SWR as low as 1.2:1 (tested with an MFJ antenna analyser).
As Ken G3WYN is licensed to transmit on 5 MHz he put out a call and before long he got a reply of 5+7 from North Hampton, Then Martin GD3YUM came in also expressing a good 5+9 from Isle of Man!
We then changed to Ken's full sized dipole and to our surprise the loop was only one or two S-points down on his full size dipole! I am currently working on a remote tuning system for the Slinky Loop antenna as it would be nice to tune it remotely. (Can anyone help with a design for a stop for each end of the vacuum capacitor drive if I use a stepper motor?)
Have a go at building this antenna. I would like to know how you guys get on, maybe we all can benefit from each others experiments.
73s, Tom
M0ZSA / ZS6SWL
If I told you I made an slinky antenna 650mm in diameter and I can transmit 80 to 17m with 100 watts with a SWR of less than 1.2:1, I think you would tell me I am smoking my socks.
In Practical Wireless, November 2003 John Heys, G3BDQ, wrote about a Slinky Hula loop antenna he made with a slinky, a hula hoop and a 120pF capacitor. So using his loop antenna design and a bit of my mechanical skills I made the following version.
It comprises of 32 turns of an original Slinky, 2042 mm of 1/2 plastic water pipe, plywood base, strong string, and the most important one 500pF variable capacitor.
I just happened to have a Jennings vacuum variable capacitor, obtained from an Amateur Radio junk sale in South Africa. This is the Rolls of capacitors, not for the faint hearted when buying a new one so if you are going to get one try and get it the same way I got mine (as cheap as possible!). I'm told even if it has lost its vacuum you can still transmit up to 100 watts(?)
If you are going to experiment with standard variable capacitors I would suggest a slow-motion drive. I found a 120pF did not tune down as low or as high as the 500pF and was very sensitive to the touch.
I started off with the pipe, marked the centre and then marked 29mm either side of the centre and then 58mm 15 times either side of the centre. I then drilled 5mm holes right through the pipe in a straight line.
The Slinky is available from many sources, mine from Maplin in Brighton (UK), though many toy stores also stock these items.
Count 32 rings of the Slinky, cut off with a strong pair of wire snips.
Screw one side of the pipe to a suitable wood base making sure it is in the same plane as the rest of the holes.
Thread the Slinky over the pipe, bend the pipe round and then screw the other side of the pipe to the wood base.
Start with tying one full ring of the Slinky to the pipe and then one on each hole ending with a full ring again (See the sketch for the circuit diagram and tie down info).
Ken G3WYN supplied a suitable knob for the capacitor as the 6mm shaft was heavy on the fingers!
The antenna tunes anywhere between 80MHz and 17MHz with a SWR as low as 1.2:1 (tested with an MFJ antenna analyser).
As Ken G3WYN is licensed to transmit on 5 MHz he put out a call and before long he got a reply of 5+7 from North Hampton, Then Martin GD3YUM came in also expressing a good 5+9 from Isle of Man!
We then changed to Ken's full sized dipole and to our surprise the loop was only one or two S-points down on his full size dipole! I am currently working on a remote tuning system for the Slinky Loop antenna as it would be nice to tune it remotely. (Can anyone help with a design for a stop for each end of the vacuum capacitor drive if I use a stepper motor?)
Have a go at building this antenna. I would like to know how you guys get on, maybe we all can benefit from each others experiments.
73s, Tom
M0ZSA / ZS6SWL