hf receiver-transmitter

[baobab72]

Dear all

during my last recurrent on the challneger i asked the instructors the reason why when we tune an HF frequency at first contact we have to press and hold the transmission key until we hear a tone - it normally takes a couple of seconds -. The answer i was given is that during that time the unit adjusts its transmit signal to the HF wavelength: as matter of fact the HF antenna is much shorter than the signal wavelength.

Can anybody shed some light on the relationship between the wavelength and the size of the antenna? does it have to do with impedance matching and if so what does that exactly mean in plain english?

is it a true statement that this process is not needed for VHF transmissions due to the fact that as the wavelength is inversly proportional to the frequency, the wavelength of VHF transmission is shorter than the wavelength of HF transmissions and thus more suitable to the size of the VHF antenna?

Many thanks

Baobab72

[winglit]

The reason is because the HF antenna is actually mechanically tuned to the wavelength selected. Well actually it is a quarter of that wavelength.

There is a device called a coupler that uses wafers of dielectric plates that are meshed with an electric actuator. These can be found in places like the leading edge of the fin or a wing. I'm not familiar with the Challenger so I don't know where the coupler is on that one.

These dielectric wafers make up the other end of a part of the airframe structure which are terminated in a notch at the other end. The airframe becomes part of the antenna. The tone that you hear when selecting a new frequency and keying the mic for the first time is to give you confirmation that the coupler has received the frequency change and is mechanically tuning your antenna, which is why it takes a few seconds.

Hope that explains it clearly enough!

[p1fel]

The front end of a transmitter requires to have the antenna matched to it's designed impedance (for example 50 Ohms) otherwise components in the transmitter front would be damaged. This is achieved by matching electrical wavelength of the antenna with the transmitted frequency wavelength.

Every frequency has an associated electrical wavelength. A radio frequency of approx' 7Mhz would have a wavelength of approximately 40 Meters. An Antenna Coupler is required using a Variable Inductor (a tune-able coil) with variable capacitors, to achieve 50 Ohms, approximately matching an antenna to an HF frequency.

Aircraft today tend to have solid state antenna couplers which match the antenna electrical wavelength in around a second or two.

Clearly a length of 40 Metres for an antenna is too long for the average aircraft airframe. Antenna's are therefore made in lengths that are fractions of the average wavelength of the frequencies most used in a transmitter. A mathematical 32nd wavelength, 64th wavelength for example or whatever the airframe would allow to be installed.

Using a Boeing 75/76 family there are two antenna’s in the leading edge of the vertical stabilizer. These antenna's are folded in a zag zag form with an approx' physical length of 18 inches each (a Boeing Techie may eventually post here more accurate details). The antenna's electrical length is longer due to the folding design. There are two matching electronic antenna couplers used to match an antenna to its equivalent electrical length of the frequency selected by the pilot to transmit.

The tone heard through the headset is a low powered oscillator transmitted carrier for the antenna coupler to match the transmitter and antenna. Transmitting on HF SSB there is no carrier transmitted unlike AM.

Some one with a more detailed and better explanation may post here later, hopefully wihout any flaming!.

[baobab72]

Dear wing
thanks for your prompt response.

is it then a correct statement that since the HF antenna is a fixed antenna and thus works for a specific ferquency, in order to match it to a difference frequency other than its natural one - 300/Mhz frequency equals wavelength -, a coupler is used to vary the length of the antenna by utilizing the airframe as an integral part of the antenna itself?

what is in plain english a wafers of dielectric?

when you key the mike and you hear the tone, that is the coupler that is matching the size of the antenna to optimize the efficacy of the system to the selected frequency, based on its wavelength?

many thanks

baobab72

[SeldomFixit]

The antenna "electrical lengths" are now electronically ( digitally ) adjusted, rather than a physical meshing/demeshing within the coupler unit.
The couplers were generally pressurised to prevent arcing and corrosion which often led to apparent failures in flight which were not apparent on the ground.
As a generalisation, the coupler was the most likely unit within an HF system to fail, moreso than the TX/RX or physical antenna.

You may benefit from reading Aircraft Radio Systems by James Powell ISBN 9780891003564

SF

[upandoffmyside]

Just speaking as an end user with previous comms experience ...

The aircraft VHF transceiver has only to work over a fairly small spread of frequency of 118 - 136 MHz or wavelengths of 2.5m - 2.2m. So the VHF antenna can be made to match the centre frequency of around 127MHz for the transceiver in impedance etc, and the there will not much be much of a change in the characteristics of the antenna as the wavelength change will only be small at around less than 10% either side of that centre frequency. I've not measured a VHF antenna length but it looks like it's around a 1/4 wavelength - around 50cm ? - which is (or was) a common antenna length.

But the HF transceiver operates over a much wider frequency band of around 3 - 20ish MHz -which equates to 100m - 15m in wavelength terms - a very significant spread. So a tuning device is required to match the characteristics of the transceiver to the antenna for efficient use every time as otherwise the transceiver will see a large variation in antenna impedance and will be very inefficient.

[aveng]

The coupler changes the apparent length of the antenna. If the impedance is not matched correctly a standing wave is formed (superimposed on original carrier but travelling in opposite direction) which reflects power back to the transmitter, which can burn out the last amplifier stage. Incidentally when you hear yourself talk through the headphones this sidetone is tapped off at this point - hence no side tone = burnt out transmitter. The reason they use 1/4 wavelength is that this is the point of maximum power in a sine wave ie 90deg. Most aircraft these days have 2 systems feeding into a notch antenna. When you transmit on HF1 for example, a relay opens in HF2's coupler to prevent the power from HF1's transmitter going straight into HF2's reciever. These relays fail from time to time as well and one HF will lock out the other.:ok:

[Comms Boy]

Hi aveng,

interesting point you make re HF antennas, i was unaware both box's utilised a common antenna system. Kind of makes sense but would be a single point of failure in its design.

I assume this isn’t the case for a twin VHF set up? The Cessna i flew years ago looked like it had 2 x 1/4 waves on the wing.

Regards
P

[shedhead]

Comms boy
It depends on the Aircraft, it was not unusual to find an antenna changeover relay on VHF comm systems at one time, less so now though. this allowed you to select upper or lower antenna for both VHF boxes with the relay wired so that if VHF1 was selected for the upper then VHF2 would feed the lower antenna and vice versa. nowadays they tend to have dedicated antennas for each VHF box. less to go wrong.

[Comms Boy]

Thanks for the info shedhead,
you learn something new every day:)

Sorry about the thread creep but, having one HF antenna system with its associated ATU would limit each HF radio connected to it to the same part of the HF band???

I guess this isn’t an issue for pilots as they probably work with just the one box and have the secondary box as a "hot standby" tuned to the same frequency???
Would be interesting to get a pilots take on how they use a twin HF set-up.
Regards
P

[hung start]

we tune the radio we are using and have the other unit dialled up with the secondary frequency . Once a unit is tuned i believe that it retains approx 10 freqs without retuning but will cycle thru these as we use 10 different freqs.When we need the other freq we will transmit for a second and if it needs to retune it will .I am happy to be corrected as needed .

[hung start]

how do i get rid of the edit tag ?

[aveng]

Once a unit is tuned i believe that it retains approx 10 freqs without retuning but will cycle thru these as we use 10 different freqs

Depends on the aircraft, old style analog couplers will still re-tune on every frequency change - new a/c digital couplers remember all frequencies that have been dialled up and work almost immediately.

[mono]

Lets just get one thing straight. The tuner (ATU) doesn't do anything to the antenna.

It "matches" the antenna to the frequency which has been tuned on the T/R.

Dual HF systems have a single antenna (older (and possibly newer) 747 excluded which has separate antennas on each wing tip) with dual ATU's. Each ATU will be tuned to the frequency selected on its respective T/R and crew can alternatively talk on either HF without a retune required.

An ATU is basically a box containing (big) inductors and capacitors and a VSWR bridge. The tone heard during the tuning cycle is (as has been said) a low power modulated RF which the ATU uses to adjust inductance and capacitance via the bridge output until it is nul and the total ATU/antenna impedance is (usually) 50 ohms. These ATU's can be mechanical or digital but the basic theory is the same.

[SeldomFixit]

Later 747's have a single antenna, in the fin L/E:ok:

[13370khz]

"Avionics For The Pilot" by Joe Johnston ISBN 978 1 86126 896 9
has a couple of pages about HF systems.

[greatwhitehunter]

Just to clarify. A standing wave is just that i.e. it does not move, the positions of it's maxima and minima, (often called nodes) are fixed. The position of the nodes and the amplitude of the wave depend on the degree and the nature of the mismatch between the feeder and the coupler/antenna.
When the forward wave from the transmitter reaches the line termination, (in this case the antenna coupler), if they have the same impedance all of the energy is absorbed and none is reflected back down the line. If a mismatch occurs some energy is reflected whose amplitude and phase depends on the type and size of mismatch. It is the interaction between the forward and reverse waves that form the fixed standing wave.

As said before the tuning is carried out at low power until the VSWR is within limits. Only then will the coupler allow the transmitter to operate at full power.

At one time aircraft like the Shackleton used trailing wire antennas which could be wound out to get a good match. Things got a bit sad if the operator forgot to wind it in before landing.

The earlier couplers used a series connected variable inductor and a parallel conected variable capacitor. The capacitor used to be made up of two blocks of copper machined into concentric intermeshing rings. The two blocks were more or less enmeshed to vary the capacitance. This capacitor was enclosed in a glass bottle and was a thing of beauty in itself. Much more fun than the new digital tuners.

[greatwhitehunter]

I'd second SeldomFixIt, Mr Powell's book is a classic for aircraft radio.:)

[Wodrick]

3rded
He taught me the basics of radar 4 decades ago - good book