Technical Information
- Page 2
3 Element Yagi
from Clear Lake Amateur Radio Club.
(http://www.clarc.org/Articles/uhf.htm)
With another DF
hunt upon us here is another quick yagi design you can try. This
is the design that I showed at DF talk a few months ago.
These antennas are relatively
small, easily constructed from common materials/tools and have surprising
performance. The feed method is greatly simpified by directly soldering
the coax to the driven element. No baluns or gamma matches are used
in this design. This simplified feed uses the structure of the antenna
itself for impedance matching. The spacing of the director and reflector
elements from the driven element directly affects the feed point
impedance of the antenna.
So, the design starts with the feed (driven element) and the elements
are built around it. Typically, a high gain antenna is designed
in the computer, then you try to come up with a matching arrangement
for a 31.9 Ohm feed! For the cost about 0.5 dB of gain, these antennas
make some design compromises for the feed impedance, use an asymmetrical
feed and make trade offs for a very clean pattern. But, they allow
simple measurements, have wide bandwidth, the ability to grow with
the same element spacing AND... you can build these antennas for
$5!!!!
The booms used for these antennas is 1/2" X 3/4" wood. The elements
have been made from silicon bronze welding rod, aluminum rod, hobby
tubing and solid ground wire with no change in performance.
144 MHz. This
antenna is peaked for 144.2 MHz but performance is still good at
146.52 (emergency use only!) Driven element dimensions are L = 38.5"
and H = 1.0" Elements are 1/8" diameter.
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144 MHz
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REF
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DE
|
D1
|
D2
|
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3 Element
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Length
Spacing
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41.00
0.00
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8.50
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37.00
20.00
|
|
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4 Element
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Length
Spacing
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42.00
0.00
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8.50
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37.50
19.25
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33.00
40.50
|
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Low Loss Current Mode Balun For 1.8 –30
Mhz
By Peter Woodland VK3ZPW
This balun / choke design will give an impedance step up ratio
of four times (1:-4:)
Great for feeding a ladder line to an all band antenna like a G5RV
or similar.
This design is a modified version of the Guanella current mode
balun that has recently been made so popular.
It will cover the whole of HF with very little loss (below my measurement
capabilities) or core heating for power levels up to around 500
watts.
Fig 1 – Coax Cable Windings
Fig 2 – Input / Output Wiring
Parts required are 4 High permeability manganese zinc ferrite
"E" cores, 2 x 300mm lengths of 92 Ohm coaxial cable, (lan cable)
and some 5 minute epoxy.
Ferrites
used are Neosid type F5, Initial permeability (ui) 1600 and a saturation
flux density (Bsat) of 470 Milli Tesla (mt) Part number 32-110-25
Any high permeability ferrite cores will
work but there may be some trade off in certain areas of performance.
Example- a pair of cores should have an effective magnetic path
length of 97mm and an effective volume of 17600mm2.. to give a 500
watt rating.
Putting it together- Glue the the two halfs of the E cores together
and press together firmly so that most of the glue is squeezed out
of the join, let the glue dry.
Wind 5 turns of the coaxial cable though the core windows, and
leave equal amounts of cable coming out. Do this on both sets of
cores, strip the ends and solder as per the diagram. The link is
only used to force a balance in respect to gnd but is normally not
needed.
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RDD
- Repeater Drenching Day
by Paul Stubbs
I, Ian, Peter (VK3ZPW) and Greaeme (VK3GRL - MDRC) went up to the
RDD repeater site to back off the transmitter audio a bit as there
was about 5db gain thru the device. The result of our last visit
and modification session. This was causing over deviation and lots
of distortion on the output. (especially during the MDRC broadcasts).
However while we were working on it there was an almighty down pour
of rain, so we quickly dropped an umbrella over the cabinet and
Ian's radio and power supply, then ran for it.  When
we looked back a few minutes later, the umbrella had blown off,
Ian's FT690 & power supply were soked (still turned on of caurse),
and there was water running out of the repeater!!! So we turned
it off, closed the doors and retreated, hoping to return Tuesday.
Maybe we should have taken notice of the weather on the way up,
I don't know how Ian could see where he was going it was that bad
(complete with lightning ete). We almost had to drive via GPS! When
we arrived it had stopped so we thought all was ok - OOPS!
PS, anyone interested in a club construction project ? (new repeater,
RT8500 etc) I have one or two ideas, however I would like some more.
(let Paul Stubbs know what facilities you would like in vk3rdd -
ed)
….a few days later…..
Today, I and Ian went back up to the repeater site. Luckily there
was no permanent damage done. The modulation level was adjusted
back to unity gain receive to transmit. Power to the antenna was
measured at 25W. And the 2 'heater' light globes were changed. Anyway,
here is a late picture from last Friday during a pause? in the rain.
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R.I.P
SO-35
In last month's Gateway I provided information on Sunsat - the "SO-35"
2m/70cm FM satellite. News to hand is that Sunsat has not responded
to commands for over a fortnight now and is thought to be inoperable.
SO-35 was the most readily accessed "Easysat" and I had several
contacts through SO-35 using a pair of handhelds with rubber ducky
type antenna's. The satellite was also popular as the passes were
friendly late afternoon / early evening.
This now leaves UO-14, which requires slightly more effort to use.
The extra "effort" is to receive the 70cm downlink. Some workable
configurations are
1. A dual band radio into a 2m 1/4 wave.
2. A satellite antenna system
3. A small UHF yagi with a 2m vertical
4. A dual band yagi
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George
Bass Diploma
All operation must meet the diploma rules and comply with WIA Band
Plans. Disqualification may occur for using FM in a SSB band segment,
or prolonged operation on DX calling frequencies. The rules are
simple:
· Operation must be two-way simplex telephony contacts across
Bass Strait, between 1 November 2000 to 30 April 2001
· Only FM or SSB modes are permitted on the 6-metre, 2-metre and
70-centimetre bands.
· Mainland stations must work five VK7 stations on a single band
· VK7 stations require 20 mainland contacts on a single band
· Diplomas are issued for single mode (FM or SSB) only
· Only one callsign may be used by each radio amateur (no multiple
callsigns)
· To claim a diploma, send a signed copy of a log of contacts,
plus $5 to: WIA Victoria George Bass Diploma 40G Victory Boulevard
Ashburton 3147
Claims received more than one month after the diploma period will
not be accepted.
CO-LINEAR
J-POLE FOR 70CM
 This
antenna exhibits approximately 5 dB over a quarter wave ground plane
antenna. Made entirely from ½" copper pipe and fittings, it features
excellent SWR over the entire 70cm band. The copper elbows and 'Tee'
sections are standard items available from any plumbing suppliers
at a very low cost. The sections here were soldered with a gas blowlamp
without difficulty.
The feed point is made from small strips of copper strap and a conventional
4:1 coax balun from RG58 coax. The length of the coax loop is 227
mm. (measured from braid to braid)
The
section below the bottom 'U' is for mounting only. It may be grounded
to a mast or ground independent. To build this you will need five
elbow fittings and one 'Tee' fitting.
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6
METRE REPEATER SERVICE & UPGRADE
Last weekend (July 2000) a large group of members visited the VK3RDD
repeater site to carry out some maintenance and other tasks. The
repeater is used each week by the Moorabbin & District Radio Club
to help extend their news bulletin broadcasts. In recent months
there has been an annoying crackle impinging on transmissions. This
was found to be caused by an intermittent in the exciter stage.
The crackle could be triggered by tapping the side of the cabinet.
Additionally, the single J-Pole antenna had a few rusty fittings
and loose rivets to be attended to.
The antenna was lowered to the ground from its 25metre high mount
and repaired.. While this was taking place John VK3XJW and Paul
VK3TGX concentrated on the transceiver equipment. The intermittent
was fixed and Paul added some new software and audio filter circuits
to the controller. The new changes will allow retransmissions of
news broadcasts without the annoying 'timeout timer' interruptions.
After everything was reassembled and tested, the group descended
on Emerald for a late 'lunch' and coffee.
The affair was a great team effort by all those present and subsequent
reports have been excellent.
The RDD repeater is located between Emerald and Cockatoo, transmitting
on 53.575 mhz and receiving on 52.575 mhz, operating with a six
cavity diplexer feeding around 25W into a single antenna. The repeater
is owned, licensed and maintained by the GGREC.
Ivan, Andrew, John, Reg and Paul relaxing afterwards in Emerald.
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Club
Net
The GGREC will now be running regularly scheduled Club Nets, on the
Club frequency 146.225.
The Nets are held on every 2nd and 4th Sunday’s of each month.
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Dummy
Load For Testing Power Supplies
Circuit supplied by Andy VK4KCS
This is a simple but useful circuit for
testing 12V shack supplies. Essentially it is a few large
transistors mounted on an even larger heatsink with a pot to adjust
current draw. Shifting the pot causes the transistors to proportionally
‘short out’ the supply under test so that a load of 2 or 20 Amps
may be selected with ease. The addition of an ammeter and
voltmeter will give the user some idea of voltage drops under higher
loads.
- The Emitter resistors are 10Watt Wire Wound
- Main Load transistors are 2N3055 or better. Add as many
as needed, although two is usually enough to dissipate 15-20Amps
for short periods.
- The driver transistor is a BD139 or similar
- The pot should be 1K or 2K wire wound
- Set pot to MIN before connecting and advance slowly.
There is a dead band until the driver turns on.
- The whole assembly can have a large bridge rectifier
placed in series with it. This will make it independent
of power supply polarity.
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Switch
Mode Power Supply Project
By Paul VK3TGX
Several months
ago I was asked to look into modifying an PC power supply to provide
13.8V to run radio gear etc. Ian sourced a cheap power supply
from Rockby electronics, this unit was duly modified and you could
squeeze 20A out of it. However the rectifier I had fitted
was quickly overheating, not due to it being under rated, rather
due to a lack of heat sinking provided in the supply. So I
started looking for a diode pack with a lower voltage drop. (ie
less heat) A friend of mine offered up a few samples to try
(he said they were better) so in went the first and -B-A-N-G- went
the
power supply!!
I was after a 3 leg double diode packs, one
of his was a 3 leg 1 diode unit! This effectively shorted
out the centre tapped transformer !
I have found some better devices from Sanyo
(90V 40A schottky) on the Internet however I have yet to find a
supplier.
Even if I do, I still have to find replacements
for the 2 vaporised transistors in the power supply - thanks Peter.
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VK3RDD
6M REPEATER UPGRADE
Notes by Ian VK3BUF
Over the Christmas break
John VK3XJW and myself carried out repairs and adjustments to the
Clubs 6 metre repeater. While the transmitter and receiver
were given a much needed alignment, the power supply needed some
work. A couple of the output transistors had died which in
turn gave intermittent repeater operation. 
John also took some windings from the power transformer which
allowed the supply to run considerably cooler.
The repeater site has changed somewhat with the addition
of an Optus cell phone transmitter hut and antenna system.
There also appears to be a commercial UHF repeater trial being
carried out on the site. (You can hear a 3khz tone on 461.625
mhz coming from an antenna on top of the tower) Neither of
these new installations seem to be having a negative effect on our
repeater performance.
It is likely that we will schedule some antenna servicing
at the site some time in the next few months.
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Please
make any suggestions or comments on any of this by email:-