Hf
 

what do we do when we press the emitting button for a few seconds before using a new HF frequency?
thanks

With radio transmitting equipment, it become necessary to tune the antena for a given frequency. This is achieved in the case of aircraft Hf transceivers, automatically. Before transmitting when a frequency change has taken place, the PTT button must be pressed. This action activates the ATU (automatic tuning unit) When tuning is complete, side tone will be re-established.

luc, HF radios in aircraft utilize a fixed length antenna that must work for all wavelengths. This requires that there be an antenna tuner between the antenna and the transceiver.

When you push the transmit button after changing frequencies you tell the antenna tuner to tune to the new frequency. The tuner compensates for the antenna not being the exact wavelength for the frequency selected.

I hope this helps you.

Check 6

thanks for the quick answer. not sure I understand everything but I will carry on doing it!!!

It does help. Thank you

Luc, each frequency in the spectrum has a "wave length." Wave length can be calculated by dividing 300 by the frequency in megahertz.

For example, 15.000 megahertz is 20 meters (wave length) (300/15 = 20 meters).

Radio signals are a sine wave. The length of one cycle of the sine wave is the "wave length."

Antennas typically are most efficient if the length is 1/4 wave length. So, ideally, the antenna length for 20 meters would be 20/4 = 5 meters in length.

This is difficult to achieve in an aircraft where the antenna must be fixed length, but different frequencies are used.

So, to compensate for the fixed length and varying frequencies, an antenna tuner "electronically" adds or subtracts to/from the length of the fixed antenna.

This is a very simplified explanation, but if you have had some trigonometry hopefully this will make a little sense.

Check 6

Hi mate.

I'm a radio ham so know all about antennas and tuning, (i think!)

You'll only really transmit between 2-13Mhz. Oceanic anyway.

The ATU (antenna tuning unit) does all the work for you. As has been posted,when you swith frequencies, just make sure the ATU is pressed before transmitting or else you will appear gobble-de-gook to ATC.!:ok:

colmac 747,
radio ham!!!

2-13 mhz?

I remember an old saying something like " its better to keep quiet and let people think you a fool than to open it and prove them correct"

Lovely words..thanks. Your point being?

2-13 Mhz is HF/Short wave..the most widely used spectrum of HF Aeronautical comms.

FE,

What is your point, 2-13 mhz covers the spectrum I've used over the years, is there something we're missing. :confused: :confused:

Dream Land

I was taught the following regarding Radio Spectrum whilst in the RAAF during trade training at the School of Radio. Rad Tech

SHF- 30-300GHZ Satelite and others
EHF- 3-30GHZ more Radar and satelite
UHF- 300-3000MHZ(3GHZ) Military UHF, Cellphones, some Radar Bands (police), Satelite
VHF- 30-300MHZ AVIATION, TV, Military Commercial FM Radio
HF- 3-30mHZ HF COMS MARITIME AND AVIATION
MF- 300-3000hkz(3mhz) Commercial AM RADIO AND NDBS
LF- 30-300khz
VLF- 3-30khz (OMEGA) Submarine Communications

basically goes up in 3 by 10 increments

Hope this helps
Sheep

Open for corrections ofcourse:p

ya that seems ok,the HF band is useable up to about 28/29Mhz,it gets very bad after that.

The short keying is to activate the ATU as already explained.
The ATU does not really 'tune' the antenna, it is a matching circuit between the transmitter and the antenna.

If the antenna is not matched (in impedance) to the transmitter then the transmitter will not produce anything like full power and the RF radiated will be quite small.

Your HF antenna has fixed dimensions which means that it has a different impedance at different frequencies.

Therefore to get the best radiated signal the ATU will be used to impedance match the antenna to transmitter each time a new frequency is selected.

ionagh,
Totally agree with your post. IN addition if you persist in keying a HF radio that hasnt been impedance matched ( tunned to its exact wavelength)via tunable ballun or impedance matching circuit, dammage will be done. The reason the outpower is weak is that the RF energy is reflected back into the Output power device PA or RF PA (Power Amplifier) the final stage of HF transmitter or any Transmitter for that matter. This inturn overheats and cooks the output device( not good, very expensive, unhappy avionics person resultant action). So tuning is very important unless there is an autotune function which inturn usually inhibits the PTT( press to talk) key until tuning is completed (in built protection).

Sheep:ok:

Maybe up to around 13MHz at the moment. Give it a few years and it will be above 20MHz. 21.885MHz used to be a 'Speedbird' (ah - BOAC!) calling frequency some 35 odd years ago when we higher up in the sunspot cycle, although that was mainly on North - South routes.

Funny to think that the job of Flight Radio Officer came and went in under 40 years, isn't it? Even Flight Engineers lasted longer...

Somebody call?:D :D

On the 'older' HF sets, the tuning circuit is actually rotating a variable tuning capacitor (blade type?) while it 'whistles'.

An antenna that isn't an appropriate fraction of the transmitted frequency's wavelength eg 1/4, 1/2 etc, reflects a proportion of the energy back down the wiring into the transmitter's circuits. Some antennas can have their length adjusted prior to use to suit the intended frequency, or to be 'close enough' for a limited range of frequencies. Some trailing wire types can be like this, and I used to use a wire dipole antenna when I was an army radio operator that would be unwound to the correct length prior to being strung.

The Antenna Matching Unit is a form of variable resistor that removes the need to change the length of the antenna. Instead it electrically simulates the additional impedance that the fixed length antenna lacks, allowing a shorter antenna than would otherwise be needed. Some types of AMUs require a short transmit burst to set themselves, others have an automatic preset related to the frequency selected.

It's not free though. The antenna will not not be as efficient as a physically correct antenna length.

The automatic HF ATU I use (yes, another one of those darn hams) has around twenty latching relays that switch in various configurations of coils and capacitors while a microprocessor tests the quality of the match with the antenna. Happens in about half a second, and sounds like a bunch of angry bees have temporarily woken up.

In theory, this can be a continual process that adjusts itself automatically without any messing around with buttons. However, the automatic matching circuit in most gear does need a steady burst of radio frequency energy to be transmitted so it can sniff what's happening and voice is just too bumpy on HF SSB for that (although smarter algorithms should cope). Hence the special button that squirts out nice solid carrier.

One analogy is that it's like a gearbox; you want to run a car engine within its most efficient range of revs no matter what the speed of the car is, so you find the right ratio to couple the engine to the wheels for the particular circumstances. That way, you get the most power out of the thing no matter what the speed. Alternatively, try going at 80mph in first gear and something will break! In the old days, Sparks had to do all the matching manually, which can be quite a complex business on valve gear where you have to watch quite a few parameters (including some that can generate plenty of real sparks), but now we've gone through the synchromesh stage to the fully automatic.

A mismatched antenna will still work, just less efficiently and - as has been said - a disasterously mismatched antenna will reflect so much power back at the amplifying transistors in the radio that they'll fry (although most modern radios should detect this condition and just grumpily shut down or reduce power while flagging their unhappiness).

It is theoretically possible to check the quality of the antenna match without transmitting (which isn't the most social thing to do) by making it one leg in an impedance bridge and tweaking for peak noise in the receiver, but that seems to have dropped out of fashion.

R

Ionagh & Sheepguts are spot on.

The last airborne HF system I worked on used to interlock the HF Transmitter with the Aerial Tuning unit: on keying the PTT the transmitter would only radiate a couple of Watts while the ATU signalled "tuning". ( The ATU needed some RF to sense the dip in reflected RF as the aerial became matched).

Once tuning was complete the ATU sent another signal to the Transmitter, stepping it up to full power.

The whole process was quite quick, (<1sec) as each time you changed frequency, the ATU would perform a coarse tune using preset aerial data. The tune sequence on PTT was supposed to just tweak the final setting (but could force a complete coarse tune if the reflected RF was too high.

regards,

KM-H

millihertz
 

FE is just having a joke

mHz is millihertz whereas MHz is megahertz

Small correction just for the sake of it...

The target for tweaking is for NO noise at all.
At that point (aka frequency at which the antenna resonates), the noise bridge will indicate precisely the impedance and reactance of the antenna.