13
$\begingroup$

On many older aircraft, and in vintage aircraft photos, you can often see long wire antennas strung from some point on the top of the fuselage to the top front of the vertical fin. On newer aircraft, it's either a rare find or not seen at all. The fact that some aircraft even today still have them leads me to wonder about their function - which radio frequency bands they are used for, whether those bands are no longer used, or whether the frequencies are still in use but the long wire has been superseded for most installations by a more compact antenna design.

Knowing that modern aviation radio operates in the 108-137 MHz band, and that most modern aircraft use much shorter rigid antennas, the long wire would seem to be inappropriate or unnecessary for that purpose. So, why would aircraft of a more modern design still be equipped with one? What other radio communication (presumably much lower frequency) would call for such an antenna?

$\endgroup$
  • 2
    $\begingroup$ They were probably High Frequency band radios, which many aircraft use for long-distance communication. HF radios have wavelengths between 20-160 meters, so antennas have to be physically bigger in most cases $\endgroup$ – SSumner Jun 7 '15 at 2:44
  • 5
    $\begingroup$ VHF (today's band) requires ground stations at visual range. In the past, stations were very few, an aircraft was most of the time out of sight of any station. The range had to be larger, and HF was used because HF signal can travel on long distances. HF antennas were long wires. Crossing oceans still require HF and SELCAL if SatCom is not used. Antenna is located in the leading edge of the vertical stabilizer. GPS has replaced LORAN. $\endgroup$ – mins Jun 7 '15 at 8:03
  • 2
    $\begingroup$ @mins I don't think SELCAL is actually required... it just makes things much nicer for the pilots, not having to listen to HF static for hours on end. But AFAIK it isn't required to dispatch overwater, and many USAF aircraft that cross "the pond" don't even have it installed. $\endgroup$ – Ralph J Jun 8 '15 at 1:43
  • 2
    $\begingroup$ @RalphJ: Correct, SELCAL is not required, and if the aircraft is fitted, an inoperative equipment doesn't prevent from taking off. Thanks for correcting. (I read on PPRuNe that Swanwick Oceanic has no SELCAL at all.) $\endgroup$ – mins Jun 8 '15 at 5:48
7
$\begingroup$

The "long wire" antenna is an HF antenna, and operates in the 2 to 30 Mhz band. Not all aircraft will use them, only aircraft that need to communicate further than a VHF radio. Typically these are found on larger aircraft including some larger helicopters. The Sea King helicopter uses this antenna as well.

It is still appropriate today as the propagation with the electro magnetic waves in this frequency band is much higher than the VHF band and allows for the aircraft to communicate part way around the world.

$\endgroup$
  • 2
    $\begingroup$ The HF bands are for transoceanic communication whereas VHF/UHF coms are used over land and are generally line of sight, At 37K ft Line of sight is approx 300 miles With HF communications are affected by daytime atmospheric signal absorption. Anything about 10MHz or above during daytime, below 10MHz during nighttime $\endgroup$ – Old_Fossil Sep 24 '16 at 8:47
7
$\begingroup$

Depending on the aircraft you could be looking at one of a few things.

enter image description here

(source)

On some older aircraft it may be the communications antenna for older style radios (pre UHF/VHF days). Most of these radios have been phased out for some time but you will see it in pictures.

The more common thing you are seeing (based on your top of the fuselage to front of the tail description), as it is still in place on many small GA planes and any bigger planes that have not had the unit removed is the ADF Sense antenna. This antenna helps to deal with the directional ambiguity problem in an ADF unit. Basically the loop antenna is capable of knowing where the station is but only on a 0-180 degree plane. It does not know if it is in front or behind the antenna. The sense antenna allows it to solve this ambiguity. These units operate in the 190-535kHz range. For what its worth the FAA is pulling a lot of ADF's and their associated approaches as GPS takes over but they seem to still be common elsewhere in the world.

Here is one, enter image description here

(source)

Why do some aircraft have them and others don't?

Since it seems like we have settled on the ADF sense antenna as the one in question it boils down to weather or not your airplane has an operational ADF installed. ADF's are still popular elsewhere in the world but here in the US much of the ADF system is being pulled as it was in part replaced by the VOR system much of which is to an extent being supplanted by GPS today. Since a large chunk of the GA fleet and to an extent the commercial fleet has airframes dating from the ADF days many of the planes still have the systems installed. Depending on where you are located, if you routinely fly one of the remaining ADF approaches or otherwise use an ADF regularly you may elect to keep it in your aircraft. I can tell you that none of the aircraft I trained in had operational ADF units although two of them had units with antennas but were non operational and simply had yet to be removed.

$\endgroup$
  • $\begingroup$ OK, I think the observation behind my question was related to the ADF sense antenna. For a time, my father owned a light aircraft which was equipped with an ADF but had no such sense antenna. IIRC, it was vulnerable to the sense ambiguity problem - you could derive a course between aircraft and transmitter, but could not be sure if it was to or from, so had to use some other means to resolve it, such as another station. $\endgroup$ – Anthony X Feb 20 '17 at 20:55
  • $\begingroup$ I second @Dave's conclusion that they are ADF sense antenna’s. They removed a long-wire antenna from mine when I had the ADF removed. $\endgroup$ – JScarry Feb 20 '17 at 22:36
  • 2
    $\begingroup$ It's also possible if the ADF system was installed in the last 30 years that they installed a combined loop and sense antenna. They are electrically tuned and have to be matched to the receiver, but they are about the size of a small notebook (20 cm wide x 30 cm long x 3 cm high). A fair number were changed out in the 80's when old units died. Getting rid of the long-wire gets rid of a lot of drag. $\endgroup$ – Gerry Feb 21 '17 at 0:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.