Monday, Dec 19, 2005 at 18:47
Just a few radio & antenna basics that may help in understanding why things are as they are with
HF radio.
There is heaps of RF (radio frequency) interference generated in cities & towns.
Firstly there are overhead power lines which can also re-radiate other stray signals. Then there is the leaky overhead cable TV / Internet systems - sounds like a chopping noise. Also interference from
home & comercial electronic equipment, traffic lights & alarm systems. Every time I go past a particular location whilst transmitting on HF I trip both a car & a
shop alarm system :-).
Video cassette recorders are another bad interference generator.
This is why we don't want any further sources of interference such as BPL etc.
Any HF VKS737 mobile antenna will be shorter than the optimum length (being a 1/4 wavelength minimum - use the formular 300 divided by the frequency in MHz for a free space measurement then multiply the result by about 95% for a real practical antenna)
Our short antennas - particulary on the lower frequencies, will radiate inefficiently - will heat up the ground with only a portion of the transmitter power going through the antenna. The best way to reduce these losses is to have a really good antenna earthing system. Counterpoise wires or radials (1/4 wave) will greatly increase the radiation resistance & efficiency of the antenna. The short antenna will also have a high angle of radiation which means good short distance contacts in particular on channels 1 & 5.
Base loading is the common method for 'loading' mobile HF antennas. The unfortunate result of this is that the loading coil will be at a high antenna current point. Better antenna efficiency can be obtained when the high antenna currents are distributed further up the antenna.
HF propagation, with the exception of ground waves, requires the RF signal to be reflected off the F layer of the Ionosphere. During the day at frequencies below 10MHz, the F layer is obstructed by the D layer. The D layer is generated by solar radiation & breaks down at night. This is why Channels 1 & 5 are best at night & channel 2 during the night as
well as early mornings & evenings.
Channels 3 and 4 are best for longer distance communications. Propagation will be influenced some waht by the solar cycle. Channel 4 should provide reasonable daytime coverage from
Melbourne to
Perth of
Brisbane. Channel 3 may be best for slightly shorter daytime contacts. Use the
Beacon call facility to gauge the best frequency / time for your location.
Ground waves are best for Channel 5 during the day & may provide a reliable coverage of up to 200KMs.
They are caused by the radio wave being bent and tilted foward by the curvature of the earth. Ground losses will determine the reliable distance of a ground wave contact.
As a summary, to improve your signal, try a longer antenna and some ground radials. Remember that the conditions of the VKS737 service & outpost station licence (RFDS) is that land mobiles are just that - not to be connected to permanent or fixed station antennas. Also remember that your improved radio signal may cause interference to other users so listen first for a quiet channel.
An improvement in antenna efficeincy will affect both transmissions & reception.
Hope that this is of some benifit.
Tony, VK3CAT, Victor 5715, VZU494
Also WIA radio examiner.
AnswerID:
144557
Follow Up By: brian - Monday, Dec 19, 2005 at 21:24
Monday, Dec 19, 2005 at 21:24
wayne,did you reduce the static much,if so what worked for you???
also has anyone fitted a noise reducing speaker,supposed to virtually eliminate background noise,but at $350-$400 a bit dear for me to experiment with...thankyou.....
FollowupID:
398145
Follow Up By: VK3CAT - Monday, Dec 19, 2005 at 22:27
Monday, Dec 19, 2005 at 22:27
Brian, Try TTS systems for DSP speakers & kits, $225.00
Don't use one here. A pair of headphones also works
well for weak signal work.
I beleive that these speakers are actually noise cancelling, meaning something like that they take a sample of the audio, phase shift it and then mix the altered & original audio back together with alot of the noise gone as it has cancelled itself out.
Cheers, Tony
FollowupID:
398159
Follow Up By: Mike DiD - Tuesday, Dec 20, 2005 at 19:55
Tuesday, Dec 20, 2005 at 19:55
Modern Noise-Reducing Speakers use DSP - Digital Signal Processing - and can do a great job of eliminating the noise and hash and make the signal much easier to understand and make listening to HF much less stressful. I use the SGC ADSP2 unit.
DSP works by converting the signal into a digital signal so that a computer can analyse the signal, identify the desired voice signal and the noise and hash, then convert the digital signal back to audio, but with much of the noise and hash removed.
Mike VK2BMM VKS737-R3701
FollowupID:
398284