News Digest 1997-2000

Az-El Rotator

Mode A Receiving Loop

VHF/UHF Yagis

WIA SA Division

A very effective broadband antenna for VHF/UHF/SHF is the simplified horn radiator. It consists of 2 sheets of metal, each forming an equilateral triangle. The length of the sides should be at least half a wavelength at the lowest frequency used. The 2 sheets are mounted like 2 opposing sides of a pyramid, just not touching at the apex, which is the feedpoint. Maximum radiation occurs towards the base of the pyramid. The input impedance at the lowest frequency is around 380 Ohms, slightly increasing with frequency. This antenna can be used with increasing gain throughout the VHF/UHF spectrum, without any resonance gaps. Thus it is the ideal TV antenna and works also on the various VHF/UHF amateur bands. Gains claimed are 7 DBD at the lowest frequency, rapidly increasing. A prototype with 2.5 m side length, the sides made out of chickenwire with 2cm gaps showed a gain of 16 DBD on 2 m. It should be noted, that for UHF and SHF the wire-mesh is unsatisfactory, and the antenna should be full metal. I myself use a 40 cm side length version of this antenna for UHF TV reception with excellent results. Being a TV antenna, this aerial does not fall under the 5 m maximum height law!

A very low profile antenna is the DDRR. Contrary to what the name suggests, it was NOT invented in the former East-Germany, but stands for "Directional Discontinuity Ring Radiator". It is a thick radiator, a bit less than a quarter wavelength long, that forms a very short vertical portion topped with a coil with 1 winding only. Thus it can be viewed as an extreme form of the "Rubber Duck". The end of the radiator is grounded. It is fed via a gamma match without the series capacitor. Overall height above ground is, wait for it, 0.00875 lambda, that is 17.5 cm on 20 metres and only 1.4 metres on 160 metres. The thickness of the radiator varies. it is around 1/1000 of the wavelength, make it a bit thicker on VHF/UHF and a bit thinner on the low hf bands. The feedpoint for the gamma match must be determined experimentally, 0.002 lambda into the horizontal portion is a rough guide. At the high voltage end of the antenna a very high quality capacitor is used to resonate the antenna at the desired frequency (that's why the radiator length had to be shorter than a quarter wavelength). Values for the capacitor range between 5 pF on 2 metres and 150 pF on 160 metres. Of course if you can tolerate narrow bandwidth (like 50 kHz on 40 metres), you can dispense with the capacitor and lengthen the radiator slightly. Needless to say, that the antenna needs excellent ground. A sheet of chickenwire extending well beyond the ring is the minimum. Construction details can be found in the ARRL Antenna Book (14th edition). This news is being radiated using 3 watts power and a DDRR antenna on 2 metres. If you like some maths to calculate your own DDRR antenna, here is a formula for the inductance of a single turn loop (which is the basic ingredient for a DDRR): L = Mu0*R*(ln(8*R/r)-2) where R = loop radius in metres, r = wire radius in metres, ln = natural logarithm, i.e. the logarithm to base e, Mu0 = 4*pi*10**(-7) = 1.26 microH/m Contributed by: Ruud, PA2RHB @ PI8WFL

This is a more conventional antenna form, a dually loaded shortened quarter wave. It consists of a vertical section, which is loaded by a coil, and an umbrella-like top, that constitutes top loading via a capacitive hat. Thus it resembles a Hills Hoist or a big mushroom.

The dimensions will depend on the following:
1. The inductance of the loading coil (the more L, the shorter the vertical).
2. The placement of the coil (the further down, the shorter the antenna).
3. The size and shape of the capacitive hat (the higher its capacity towards ground, the shorter the aerial).

The ARRL Antenna book 14th edition shows a 40 m antenna being 1.80 m in height with a hat diameter of 2.10 m (8 spokes of 1.05 m length each, connected by wire at the outside). The loading coil is near the top. The vertical section is insulated against ground. An L/C network is used for impedance matching. The system is placed on a small ground mounted metal disk with 60 radials attached to it. These run just above ground.

Possible variations:
1. It is simpler to ground the antenna and couple the cable via a gamma match (forget about the series capacitor - it is unnecessary).
2. Instead of the 60 radials a mat of chicken-wire could be used - the bigger the better.
3. The loading coil could be brought closer to ground, decreasing the number of turns required. Be however aware, that the coil must be of very high quality wire. The closer to ground it is, the higher the currents induced will be. The efficiency is claimed to approach a full size vertical. The bandwidth is relatively narrow: 50 Khz on 40 m for an SWR of 1.5 or less. Again it must be stressed, that the dimensions given are not binding, and can be adapted to suit your local environment. To gain maximum bandwidth, use as much height as possible. To gain maximum performance, use the best ground (radial) system feasible.

Loaded Verticals

1. Top loading by a capacitive hat can be achieved by various means: An umbrella like configuration like the one with the Mushroom antenna. This can be made to slide up or down the vertical portion, thus enabling to tune the resonance throughout the band. This is particularly important for short verticals. Another possibility is to run a horizontal portion of wire at the top; that's for instance how you can use a 20 metre long Windom antenna on 80: By using the feedline as a loaded vertical.
2. Base loading is achieved by a coil at the bottom of the antenna.As with the Mushroom antenna, this coil must be of very high quality. My first QSO on 160 ended, when my loading coil for a 7 metre vertical caught fire! And this with 100 Watts output only! It is questionable, if putting the loading coil higher up is of much help: You can get away with a lower quality coil, however you will have to increase the number of turns dramatically, the higher you go with the coil.
3. Continuous loading can be achieved by using a "rubber duck" on the hf bands. I have seen a 2 metre long 160 metre antenna. The bandwidth however will suffer greatly with such a contraption.

1. Horizontal antennas can be hung below 5 metres. On wavelengths higher than 10 metres this will direct maximum radiation towards the zenith. This need not be a bad thing; for satellite communications it's actually useful, also on the 80 and 160 metre bands for local traffic.
2. Sometimes a "Raingutter Antenna" works quite well: A wire attached to your guttering and tuned with an ATU. Contrary to what some antenna-books suggest, you need not insulate the downpipes. I used such an antenna for years at my old QTH in Sydney. The downside is, that it picks up any electrical noise generated in the house, such as computer-hash, very well. Thus a small additional vertical for receiving purposes might be advantageous.
3. Various antenna books describe methods of concealing antennas, like using thin, coloured wire. Well, if such an antenna is higher than 5 metres, you will still legally require a permit, though it might take longer to catch you, if you haven't got one. Thus we cannot recommend this procedure.
4. I tried an indoor antenna whilst staying at a Motel in Morwell in VK3 and worked stations from VK2, VK4 and VK5 on 80m in SSB without really trying too hard. Amongst the QSOs was a rag-chew of over an hour's duration! The trick is to run a wire from the antenna end of the ATU as high up as possible, say to a curtain track and drop the end of the wire (possibly rolled up) into a far corner of the room. As counterweight take a similar length wire and clamp the far end to something resembling earth, say a metal window frame. With suitable coupling plugs both wires can be made out of regular coaxial cable. If you find it impossible to couple the antenna to the radio, lengthen it, until you can. Of course this will only work if the motel room does not have a metal roof. If it has, 80m is out, and you must use higher bands. Of course it's all the better if you can install such an antenna at home under the roof!
5. If all else fails, there is always portable operation. Get out in the bush, install temporary antennas, and have more fun than at home! If you ever have participated in the John Moyle fieldday, you'll love it. For best results on HF look for a site with high groundwater level, say next to a river or lake. For best VHF/UHF results look for mountaintops with little or no vegetation.

Bob, ZL3NE writes:
Gentlemen. For the last four years I have carried out an extensive study
on VHF propagation, here are the results. Now before anyone rushes into
print let me ask them to, " try it out for say twelve months", I am sure
they will be surprised.

I kept a record of every opening, the time of day and the weather
pattern applicable when the opening took place. From an area such as
Auckland, propagation can only cover 180 degrees, and during an average
year, openings should occur on 6 meters, 50 days per year. I set out to
do three years, but after 28 months my permit had been cancelled which
slowed things down a bit, still I had recorded enough openings to make a
51-day average per year, without the last eight months.  For the 28
months of the study, the following propagation took place as follows,

6 meters Auckland to         VK2          VK3       VK4        ZL4
Along frontal zones ----------40 -----------6 -------57 ------- --
Across frontal zones --------- 3 -----------3 --------4 --------12
Across anticyclones 1024+ ----36 ----------15 -------42 ---------7
Total number days open--------79-----------24-------103 --------19

It was noted that propagation only took place, when one of the following
weather patterns was active in the propagation path.

Intense anticyclones.
====================
These produced propagation when the pressure exceeds 1024 hPcls in
summer and 1040 hPcls in winter. Signals can vary from very weak to
quite strong, and depending on the air pressure and air temperature
combination, propagation can reach 1296 MHz. The higher the pressure,
and the hotter the average temperature, the higher the frequency of
propagation.

Warm and Cold fronts /Ducts.
===========================
These are found where the isobars are very close together, such as in
the warm winds preceding warm or cold fronts. They quite often coincide
with jet stream winds, and often contain CB (or thunder). They are
prominent on weather maps shown in the daily papers, TV maps, and in
weather fax maps, even ducts have been displayed there. From these
weather maps, you will be able to pick out the paths of propagation and
see the principles I have shown. Signals can be extremely strong with
2-watt stations being able to contact on 2 meters to 2250 km. As with
the previous mode the higher the temperature, the higher the MUF goes.

Across fronts.
=============
With a temperature change of say 10+ degrees. That is, from the warm
side of a front, to the cold side. The angle to cross these fronts is
above 70 degrees, But if the front contains a very active thunderstorm,
then from any angle. Under these conditions, back-scatter is very
prominent with skewed propagation as well.

This year I have been studying, weather types, during which NO
propagation has ever been recorded here. They are: -

1. Anticyclones with air pressures below 1024 hpcls. (Christchurch has
enhancements, which give propagation to 1020 hpcls).
2. Occluded fronts.

Now let us compare this season, Nov, Dec, and January 92 days?

In Auckland, occluded fronts were recorded on 29 days. Openings recorded
Nil.

Weak anticyclones 1024 hPcls and below, were recorded on 44 days,
propagation, Nil.

Intense anticyclones over 1024 hPcls, recorded on 7 days. Propagation on
all 7 days!

Fronts and tropical cyclones were recorded on 12 days; propagation took
place on all 12. If you wondered why Auckland signals were scarce you
now know why.

Propagation seasons are altered by the sun changing, the air temperature
to air pressure, and the relationship / quantity, of frontal activity.
As an example, an anticyclone in summer of 1025 hPcls with an air
temperature of 30c will produce propagation from VK to ZL, in winter
with an air temperatures of 15 c we need an anticyclone of 1040 hpcls!
See the relationship.

Propagation is always evident here on ten meters before showing up on 50
MHz. Therefore 10-meters can be used as an indicator of when the
propagation is going to open to 50 MHz. and higher. The first indicator
to look for in Northern New Zealand, is the VK2RSY beacon on 28.261 MHz,
it is always in first, and when it reaches S7 on my S meter the
six-meter band is open. From VK2, 3, 5, 7 use the ZL3 beacon on 28.228.
From VK2, 4 use ZL6B on 28.200. Another interesting beacon for me is the
JA5 on 28.264 MHz. When this reaches S7+, six is open to Japan. In all
my findings I have always had 10 meters open first, then 6 meters. When
6 meter signals are S9+, I find 2 meters opens, when 2 meters reaches S7
or above, 70 cm opens.

Could I suggest that an approach be made to your local TV channel with a
request that any major CB activity within 1500 km of your area, or in
any propagation path be shown or announced in their weather maps /
reports. I have done that here, they were surprised anyone was
interested, but were willing to co-operate. Repeatedly I am advised of
CB coming before it provides propagation. May I say, always look ahead,
and try to improve your percentage of opening strikes. This is what
Amateur VHF DX is about.

Gentlemen this is a brief run down on the subject. Take the next year,
and see how you go with it. See if you can increase your number of
contacts, good luck. Bob ZL3NE.