Monthly Archives: July 2013

HF Antennas & Feeder Lengths in Metres

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This handy chart was developed by Gary, ZL2IFB. It is a very simple way of finding out what length of wire you need for your particular frequency. Thanks Gary!

Band CW SSB ¼ wave ¼ – 5% ¼ x 0.66 ½ wave ½ – 5% ½ x 0.66 Full wave F – 5% F + 10% F x 0.66
160m 1.825 41.07 39.01 27.38 82.13 78.03 54.76 164.27 156.06 180.70 109.51
1.850 40.51 38.49 27.01 81.02 76.97 54.02 162.05 153.95 178.25 108.03
80m 3.520 21.29 20.23 14.19 42.58 40.45 28.39 85.17 80.91 93.69 56.78
3.800 19.72 18.74 13.15 39.45 37.47 26.30 78.89 74.95 86.78 52.60
40m 7.025 10.67 10.14 7.11 21.34 20.27 14.23 42.68 40.54 46.94 28.45
7.100 10.56 10.03 7.04 21.11 20.06 14.07 42.22 40.11 46.45 28.15
30m 10.105 7.42 7.05 4.94 14.83 14.09 9.89 29.67 28.18 32.63 19.78
20m 14.020 5.35 5.08 3.56 10.69 10.16 7.13 21.38 20.31 23.52 14.26
14.250 5.26 5.00 3.51 10.52 9.99 7.01 21.04 19.99 23.14 14.03
17m 18.075 4.15 3.94 2.76 8.29 7.88 5.53 16.59 15.76 18.24 11.06
18.120 4.14 3.93 2.76 8.27 7.86 5.51 16.54 15.72 18.20 11.03
15m 21.020 3.57 3.39 2.38 7.13 6.77 4.75 14.26 13.55 15.69 9.51
21.250 3.53 3.35 2.35 7.05 6.70 4.70 14.11 13.40 15.52 9.41
12m 24.900 3.01 2.86 2.01 6.02 5.72 4.01 12.04 11.44 13.24 8.03
24.950 3.00 2.85 2.00 6.01 5.71 4.01 12.02 11.41 13.22 8.01
10m 28.025 2.67 2.54 1.78 5.35 5.08 3.57 10.70 10.16 11.77 7.13
28.500 2.63 2.50 1.75 5.26 5.00 3.51 10.52 9.99 11.57 7.01
6m 50.100 1.50 1.42 1.00 2.99 2.84 1.99 5.98 5.68 6.58 3.99
52.050 1.44 1.37 0.96 2.88 2.74 1.92 5.76 5.47 6.34 3.84

Where a full wave in metres = frequency in Hz divided by the speed of light (299,792,458 m/s)

Reduce dipoles by ~5% for end effects. Add 5-10% to full wave loops. Trim all antennas to resonance.

Lose one third physical length for velocity factor of typical coax. 73 de ZL2iFB

Cycle 24 Disastrous for Hams

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The sun is currently in the peak year of its 11-year solar weather cycle scientists say.

This year’s solar maximum will probably be the weakest in 100 years and the next one could be even worse. The sun is currently at the maximum of Solar Cycle 24, but as this graph shows, there are far fewer sunspots during this peak than there have been in past cycles.

Cycle 24 in July 2013

Cycle 24 in July 2013

Solar Cycle 24 has been underway since 2011 and its peak was expected in 2013, but there have been fewer sunspots observed this year compared with the maximums of the last several cycles. The stage is set for an even smaller maximum during Cycle 25 around 2025.

A small Cycle 24 also fits the 100-year pattern of building and waning solar cycles as witnessed by scientists during the weak solar cycles at the beginning of the 19th and 20th centuries.

DX is always there. Even in the low times of the sunspot cycles. It’s just that you will have to be a bit more cunning to work it

I cannot remember a longer, more tedious cycle than this one and it’s the sixth one I have been in since 1958 although I did manage to catch the end of grand daddy of them all which started in 1948 and finished in 1952. The first big one for me was 1959-1961 when you could work the world on 10 metres day after day with low power. Sometimes the band was open 24 hours. This latest Cycle 24, except for a a few good bursts last year has been a big disappointment.

I will stress again that the DX is always there. Especially when a contest occurs, they all come out of the woodwork!

And that’s DXing folks!

73, Lee ZL2AL

Plasma TV RFI Cure by K9YC

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Here are the main points.

1) If it IS the TV, SOMETHING connected to it is acting as a 160
meter transmitting antenna and radiating it strongly enough for your
receiving antennas to pick it up.

2) There may be more than two antennas. Prime candidates are whatever
that TV is using as a signal source (cable TV, satellite, roof
antenna), the power line, and even connections to the audio system.

3) Like any other 160 antenna, bigger makes a better transmitter, so
use your noggin in figuring out which of these antennas are most
likely to be the most effective long wire antennas.

4) Apply BIG ferrite chokes to those antennas (not your antennas, the
ones connected to the TV). What you are looking for is the greatest
possible impedance from the choke on the frequencies where you are
hearing trash. See the tutorial on my website to learn how to get
that high impedance. It includes graphs of lots of MEASURED data
contributed by another member of this list who did them in a very
good lab.

http://audiosystemsgroup.com/SAC0305Ferrites.pdf

While the tutorial is written for pro audio people, any decent RF guy
should be able to use it to understand, troubleshoot, and fix ham RFI
problems. I’m working on a tutorial dedicated to ham radio
applications.

5) You need Z on the order of several K Ohms to make a serious dent
in this trash. As the data shows, for 160, you are wasting your time
(and money) with anything less than 10-14 turns around a 2.4″ #31
toroid, or 7-8 turns through one of the biggest #31 clamp-ons.

6) Do NOT try to kill the noise by finding “a better earth
connection” for that system. Any noise that you shove into that
ground wire will radiate, just like any other antenna carrying RF
current.

7) If you are going to put a commercial AC line filter on the set,
make sure that you have the shortest possible connection between the
filter and the set, ESPECIALLY for the green wire. See #6 above.

8) 12-14 turns of twisted pair like THHN stranded around that #31
2.4″ toroid makes a very good common mode choke for 160-40 meters.
Run only “hot” and neutral through the toroid, carry the green wire
around it. Put an RF cap of at least 0.33 uF between line and neutral
on the power source side of the choke. This cap will form a
differential mode filter with the imbalance in the inductance of the
choke. It must be rated for the AC line voltage. The wire in this
choke doesn’t need to be very big — the vast majority of standard
IEC power cords are #18, even those that look a lot beefier.

9) Do NOT add capacitance between hot or neutral and the green wire.
In addition to #6 above, doing so would create more leakage current
at power frequencies than is permitted under safety codes.

73, Jim Brown K9YC