hf 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

[The Fat Controller]

Wavelength (m) = 300 / frequency in Mhz

On VHF the radio frequencies vary from 118-136 Mhz or so, therefore wavelength varies from 2.54 to 2.2 m.

VHF antennas are often 1/4 wave, so a 60 cm antenna is suitable, this can be helical or folded in a casing to minimise protrusion.

This stubby little VHF antenna can cope with this range of frequencies without any tuning in the transmitter.

On HF, say one frequency is 2.800 MHz and the backup 8.500 Mhz, wavelength now varies from 107 m to 35.3 m.

An good HF antenna is about half on the wavelength, so either 53 m or 18 m is required.

If you have a fixed HF antenna on your aircraft then some tuning of the antenna circuit is required to accommodate the large variations in wavelength due to the lower frequencies used.

The alternative is to use a trailing wire that varies in length with the change in frequency.

Not sure what kind of antenna you have, but when you press and wait for the tone that is the adjustment taking place.

[baobab72]

Dear
many thanks for your prompt answer.
i have been in aviation for almost 20 years and i have never heard such a comprehensive explanation about antennas and wavelength.
To clarify: the hf comm system of the airplane that i fly - the cl605 - is comprissed of two transceivers and of two antenna couplers operating within the HF frequency range between 2 and 30Mhz. The single antenna is engulfed within the leading edge of the vertical stabilizer, i am not sure of the type of antenna or of its length.
if the vhf antenna is 1/4 of the wavelength, what about the hf antenna?
when you key the transmission key, what does it mean that the vhf transmitter is adjusting to the signal? maybe that it is multiplying the signal to the length of the antenna?
sorry for my poor wording but i am pretty far from a frequency expert!!

baobab72

[The Fat Controller]

Your antenna is a FIXED size, so therefore it's "natural" frequency is also FIXED.

Your transmitter will transmit on whichever frequency you select.

In order to "match" the transmitter to the antenna, you have the coupling unit which is what will be adjusting capacitance and inductance to make sure no power is reflected back from the antenna.

More information here

Antenna (radio) - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Antenna_(radio)))

Happy reading.

I am an ex merchant navy radio officer and to match our antennas over a wide HF frequency range we had a cabinet the size of a refrigerator with winding handles, this was our version of your coupling unit.

I now work as an ATCO at Scottish Control, so just push my transmit button to talk to you guys, leaving the engineers to worry about the systems.