Hf

[luc]

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

[Formally Known As]

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.

[Check 6]

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

[luc]

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

It does help. Thank you

[Check 6]

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

[colmac747]

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:

[FE Hoppy]

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"

[colmac747]

Lovely words..thanks. Your point being?

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

[Dream Land]

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

[Sheep Guts]

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

[EI-MICK]

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

[ionagh]

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.

[Sheep Guts]

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:

[radeng]

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...

[BOAC]

Somebody call?:D :D

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

[Tinstaafl]

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.

[Self Loading Freight]

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

[KM-H]

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

[enicalyth]

FE is just having a joke

mHz is millihertz whereas MHz is megahertz

[iakobos]

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
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.

[radeng]

iakobos,

small correction......

The antenna itself is rarely resonant: the tuning circuit provides an impedance transformation such that the correct load is presented to the transmitter (usually 50 +j0). This may appear academic until you have to start working out how much power is radiated (or more usually, turned into heat!)

The argument as to whether or not the transmitter should match the load is a different one which has occupied technical journals in vituperative correspondance, and I won't raise that here!

[iakobos]

Radeng,

You did not correct me, you just added a point intended for the non-experts. ;)

An antenna is always resonant somewhere, the problem is, nowhere else.
Therefore the TU or ATU that will fool the transmitter to think he married the right bride.

[BEagle]

When I were a lad, we thought that maximum deflexion on a meter was A Good Thing. So the ATU ('Wavemeter') attached to our school ex-WW II Wireless Sets No. 12 ('Twelve Set') was always adjusted to give peak meter values.....

That was in 1966.

15 years later, along comes (legal) CB radio in the UK. I learn about the mysteries of VSWR and tuning antennae for minimum SWR. After much tweaking and adjusting, I obtain a good match across...err, 80 channels in the 11 metre band in my car with a 'disguise' base-loaded 1/4 wave whip.

Which rather explained why there were so many complaints about the interference our dreadfully mismatched longwire antenna used to cause at school! And no wonder it was so difficult to contact other stations on our 5 HF spot frequencies (on or about 2-7 MHz. Or Megacycles as it was back then!)

A little knowledge is a dangerous thing!

[iakobos]

BEagle,

Your post exposes two shocking facts.
One you were a pirate, two you were an English pirate. ;)

73's

[BEagle]

Nope - it was quite legal to use the school shortwave transmitter and receiver on the official frequencies assigned for use by the Combined Cadet Force and Army Cadet Force.

And the FM CB frequencies were also legal; 40 channels were approved in 1981 and another 40 a few years later. Although I do admit to setting up the antenna to cover the whole frequency spectrum approved for use in the UK before the lower 40 channels were released!

88s (no tongues!)

[GrahamCurry]

73 and 88 should NEVER be used in the plural form

[iakobos]

Right,

In true Western Union language then:
23 73 30

[radeng]

iakobos,


>An antenna is always resonant somewhere, the problem is, nowhere else.



There's a nice area for argument! Depends on the antenna, and what you mean by resonance......e.g. a cavity backed spiral antenna used for Electronic Counter Measures is a reasonable match from 2 - 18GHz. Where is it resonant?

Sadly, the older I get, the more I realise that I know less and less about radio engineering, when it really ought to be more and more.....

[iakobos]

Radeng,

This is an enhanced aperiodic dummy load, not an antenna. ;)
With enough power injection it works, just make sure it's properly cooled.

[radeng]

Iakobos,

It certainly receives......But what about the whole family of travelling wave antennas? Log periodics (which you can argue use multiple resonances, although I'm not sure about the Green's equation boundary conditions), terminated rhombics and Vee beams, even Discones? Slots are another case, too - not sure how widely used these are on aeroplanes at HF these days.

Getting a bit academic for this fora.......

One problem is definitely that things you'd like to do for antenna efficiency on aircraft don't always sit well with either aerodynamics or structural requirements.

[iakobos]

Every good rule has an exception, hence the good rule.
Aerial systems (much more poetic than antenna isn't it ?) have their exception.

TW are not used for transmission.
Rhombics (terminated or not), vees (ditto) are resonant antennas, which can be used on a wide spectrum provided there is a matching device, in which case they are used in the non-resonant "mode", usually with a purpose (directivity/gain).

Discones are the poor man's antenna and the scanner delight...
Nowhere that good but nowhere really that bad.
Usually not intended for transmission either.

Terminated folded dipoles are resonant antennas to which a nice resistor has been added. The most sophisticated dummy load so far. ;)

If aircrafts could just stay on the ground it would make radio engineers so much more happy and the quality of communications would be greatly improved. What can you do ???

[radeng]

Iakobos,

>If aircrafts could just stay on the ground it would make radio engineers so much more happy and the quality of communications would be greatly improved.


Yes, we could use plug in telephones and get reliability! Having made a living for over 40 years designing radio systems, I maintain that the use of radio should be the last resort, especialy where you want reliability..........