MARTIN - G8JNJ

You can only consider yourself as being a real radio amateur if you view every object around you as being a potential antenna !

Here's a selection of some of the designs I have built.

Rotary washing lines can be obtained from re-cycling centres for a few pounds, and can almost be considered as a 'Stealth' antenna.

They can be used for all sorts of purposes. 

Here's a Petlowany Site 1 or Site 2 loaded vertical I made for 3.6MHz built from two washing lines back to back.

The base is a four arm drier made by 'Hozelock', this company seems to manufacture some of the better types I have aquired. 

The top drier is of unknown manufacture, but it is very light weight and both of the fastenings to the centre pole can slide up and down and be locked in position with large nylon screws.

I used three wire spirals in this design which made it resonant at around 5MHz, however I intend to change this to just one long spiral, which should make it tune directly on 3.6MHz. 

Here's a closer view of the spiral top loading coil.

 

In order to bring it to resonance on 80m, I had to cut the aluminium support tube at the base and insert a short length of PVC water pipe to add a matching coil.

 

Note the bonding straps made from coax braid which I added to ensure all the metal bits were at the same potential.

Tent pegs at the ends of the arms help stabilise it in high winds, and improve the grounding. Adding additional counterpoise or radial wires further improves the performance

 

Hmmm. I think I could turn this into a small rotatable loop by adding some fishing pole spreaders.

Perhaps a GM3VLB Cobwebb or Hexbeam

 

You can also use them as bases for other antennas, such as this 10m fishing pole vertical.

I found these washing lines in a local DIY store, but they are also available on e-bay.

You can easily open them by holding the reel in the palm of one hand with the winding spool facing upwards. Grip the plastic surround around the reel with the other hand and rotate it anti-clockwise to unscrew the two halves.

Replace the washing line with some thin wire. With care, I can get about 33m / 100ft of 0.7mm dia wire on a single spool.

Keep the wire under tension by running it through your fingers when winding the wire back in. This makes it spool more evenly.

A croc clip on the end of  the wire makes it useful as either an antenna or counterpoise.

You can tune it by winding the wire in and out, and then locking it off by wrapping the wire around the tab on the spool.

Two or more of these are very useful for portable operation.

The following is a prototype antenna I'm currently constructing. 

 

Note that 28mm Dia pipe and octagonal shape (use 45 degree elbows) would give approx 2dB more gain at 7MHz but would cost about £15 more in materials (at current prices).

 

 

Construction- progress so far:-

 

I have soldered up the loop and hinge assembly and checked it with a temporary feed loop and no tuning capacitor.

 

I wanted to ensure that in it's basic form the loop was resonant above 29MHz, as I was concerened that the end capacitance with the hinge assembly fitted would be too great to allow operation on 10m.

 

 It looks like the design parameters I selected were OK as the resonance is at 31MHz.

 

 

I decided to mount the brass hinges by cutting a slot in the end of the pipe and soldering them directly in.

 

 

The next stage is to fit the tuning plates on to the hinges and make a perspex plate to mount across the top faces of the hinges to hold them apart at the correct spacing. I may also have to use a spacer between the tops of the hinged plates to stop them shorting as they swing open. Some short straps made from coax braid will connect between the two halves of the hinges in order to ensure a good low impedance moving joint.

Many amateurs have at some stage experimented by using ‘Slinkies’ as antennas. Often with varying degrees of success.

 

I performed some experiments with a ‘Springy’ which is the most common version found in the UK.

 

 

http://www.hawkin.com/ search for ‘Springy’

 

This comprises of approx 75 turns of flat steel strip wound so that it has an outside diameter of about 70mm. The strip is about 0.5mm thick and about 1.5mm wide.

 

My original interest was for use as a portable antenna. I have a 9m glass fibre fishing pole which I was using as a multiband vertical with a 9m vertical wire attached to a 4:1 balun at the base fed from an LDG-Z11pro autotuner.

 

Although this gave good results on the HF bands I wished to also operate on 80m so I was looking for something which would be electrically longer than the 9m wire, which could be supported in the same way but would still be easy to deploy. The ‘Springy’ seemed to be a good option.

 

The first tests involved stretching the ‘Springy’ to various lengths by securing it to the top section of the pole, with the pole running through the centre of the ‘Springy’ The mast was then extended section by section and whilst I measured the it’s natural resonant frequencies.

 

The following table shows the fundamental and other resonant frequencies with the ‘Springy’ stretched to different lengths.

 

 

Length	F1 -  MHz	F2 - MHz	F3 - MHz	F4 - MHz	F5 - MHz
1 section 1.3m	3.4	7.1	24.5	39.2	51.1
2 section 2.3m	2.9 - 3.8	7.8 - 7.9	24.2 - 24.6	39.9 - 40.5	52.7 - 53.4
3 section 3.3m	3 - 3.7	7.5 - 7.7	23.3 - 23.8	39.2 - 39.8	53 - 53.7
4 section 4.3m	3 - 3.5	7.2 - 7.6	22.4 - 22.9	37.8 - 38.4	51 - 52.3
5 section 5.3m	3 - 3.4	6.9 - 7.2	21.6 - 22.2	36.4 - 37.1	50
6 section 6.3m	3 - 3.3	6.4 - 6.8	20.9 - 21.4	35.1 - 35.2	48
7 section 7.3m	2.8 - 3.2	6.1 - 6.5	20 - 20.3	28.8 - 29.8	43 -44
8 section 8.3m	2.8 - 3.1	5.9 - 6.3	19.4 - 19.9	28.4 - 29.1	42 - 42.5

 

The green blocks show useful resonances in various amateur bands.

 

Some variation in the actual resonant frequencies occurred as the ‘Springy’ moved around in the wind. This was particularly noticeable at the lower frequencies where the impedance was almost constantly changing. This was due to changes in inductance and capacitive coupling between the closely spaced turns. This effect reduced as the ‘Springy’ was stretched and so the turns were held steady under progressively greater tension.

 

The next test was to compare the field strength of transmissions first using the 9m wire and Balun / Autotuner and then using the ‘Springy’ stretched to the same length.

 

To make the measurements I used a remote controlled Icom PCR-1000 and Datong active antenna located about 2 miles away from the transmitter site. I also used a spectrum analyser and short vertical antenna about 30m away from the transmit antenna in order to try and verify the results.

 

Frequency MHz	1.92	3.62	7.06	10.1	14.14	18.11	21.115	24.94	28.55	50.165
Springy (Local Spectrum Analyser)  level in dBm	-48	-55	-35	-39	-38	-50	-33	-40	-30	-45
9m Vertical (Local Spectrum Analyser)  level in dBm	-50	-44	-33	-38	-37	-40	-40	-36	-30	-50
Springy gain relative to vertical dB	2	-11	-2	-1	-1	-10	7	-4	0	5
Springy (Remote receiver) level in dBm	-103	-72	-78	-87	-75	-84	-88	-85	-94	-110
9m Vertical (Remote receiver) level in dBm	-105	-70	-76	-85	-72	-82	-82	-80	-91	-110
Springy gain relative to vertical dB	2	-2	-2	-2	-3	-2	-6	-5	-3	0

 

 

In most cases the ‘Springy’ provides much worse performance than the vertical wire. The results obtained with the local spectrum analyser should be treated with some suspicion, as nearby objects and cables can produce large variations in the RF field strength. Measurements made with the receiver at some distance are a much better indicator of the actual performance.

 

Note that the longer electrical length of the ‘Springy’ makes a big difference on 1.9MHz, but the losses become greater as the frequency increases. I believe the ‘Springy’ figures at 50MHz are better due to the 9m wire being over 5/8 wavelength resulting in a very high angle radiation. The ‘Springy’ may only be radiating from the first few turns, producing a lower angle of radiation.

 

As can be seen the general trend is that the ‘Springy’ is about 2 or 3 dB worse than a straight wire. The other problem is that the ‘Springy’ moves around a lot and so the impedance is constantly varying. I had hoped that it would be possible to stretch the ‘Springy’ to achieve a good match without using an autotuner. But this was not possible unless the ‘Springy’ was held perfectly still.

 

So the high losses of the steel ‘Springy’ and its mechanical instability don’t seem to make it particularly useful for antenna construction. I certainly wouldn’t use it in place of a wire dipole, unless I had really bad space restrictions.

 

I also would not recommend using one as a loading coil, as resistive losses are too high. I tested one for use in an ATU and measured 3dB loss across just ten turns .

 

However, other versions are available which are manufactured from less lossy materials (or are plated) and which may be more mechanically stable. But so far I have not had an opportunity to experiment with these.

 

Martin Ehrenfried - G8JNJ 21/05/2008 V2.3