Mr Spaceman
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Strange Looking Antenna On B-727?

Sat Jan 12, 2002 2:55 am

Hi guys.

While doing a photo search of airliners I have flown on, I came across a Royal Airlines 727 that I flew on from Toronto to Halifax in December of 1998.

In the first photo below, you can see a strange looking antenna on the belly of the 727 behind the nosegear. It looks like 4 antennas attached as 1.

What type of antenna is this? Is it for NAV, COM, etc ? Perhaps it's an older radio altimeter antenna. I suspect it's for older technology equipment because the cockpit photos that I have of this 727 (which were rejected) show only "old" instruments.


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Photo © John Kelley



Here's one of her sister ships below, without this strange antenna.


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Photo © Dave Kirkby



Chris  Smile
"Just a minute while I re-invent myself"
 
airplay
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RE: Strange Looking Antenna On B-727?

Sat Jan 12, 2002 3:08 am

This is most probably an HF antenna for the "High Frequency" communications systems.

Not all aircraft are fitted with HF. It depends on the operational capabilities.

Many HF antennas are not as obvious as this one is. On large aircraft they can range from a spear (like on the tail of a 707) or on a wingtip, or they can be part of a fairing, or they can be a long wire attached between the vertical stab and the fuselage (like on many 737 aircraft).

Like usual when it comes to airplanes, there is no single solution or configuration. This 727 just happens to have a "towel bar" HF antenna.

 
Mr Spaceman
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RE: Strange Looking Antenna On B-727?

Sat Jan 12, 2002 3:24 am

Hi Airplay.

OK, Thanks for your answer. I've never seen an antenna shaped like this before so I became curious. The nickname "Towel Bar" for this HF antenna sure is apropriate. You can see the spear shaped HF antennas on the wingtips of 747-200s very clearly...like the one below.


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Photo © Frank Schaefer



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exPratt
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RE: Strange Looking Antenna On B-727?

Sat Jan 12, 2002 4:12 am

Isn't that stinger off of the tailcone in lower picture of the Royal 727 also an HF antenna. As Airplay stated, there are many different configurations, apparently even within the same type of airplane.
 
Mr Spaceman
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RE: Strange Looking Antenna On B-727?

Sat Jan 12, 2002 4:23 am

"Good Eye", Expratt.  Big thumbs up I don't know the answer myself for sure, but from what these more knowledgeable fellows have been teaching me/us, I'd say it certainly looks like a HF spear/stinger antenna.

Someone will let you know for sure.

Chris  Smile
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JohnM
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RE: Strange Looking Antenna On B-727?

Sat Jan 12, 2002 11:49 am

I have seen ADF antennas look something like that, but on the top of the fusalage.
 
Mr Spaceman
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RE: JohnM

Sun Jan 13, 2002 4:07 am

Hi JohnM.

Here's a pix of an antenna that looks similar to the one on the 727. Is this the same type that you were talking about? It's on the belly of the C-172 below the registration. I'm not sure what type of antenna this is. It could be an ADF or maybe even a VHF.


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Photo © Ariel Shocron


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TechRep
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RE: Strange Looking Antenna On B-727?

Sun Jan 13, 2002 10:35 pm

That is an HF quick MOD. When aircraft are purchased, typically from US carriers, that didn't have the HF installed in the vertical stabilizer as an option from the OEM, they will install this ugly HF antenna. The foward baggage compartment is disessembled and wire strung from there to the E/E Compartment then to the flightdeck. This is a cheap alternative than to wiring all the way back to the tail. I do not know who has the STC to do this but have seen many installed at many Part145 repair facilities.

The other pictured antenna "shark fin" are VHF1, VHF2 and VHF3. This aircraft has no GPS antenna installed I can see. Just in front of VHF1 is a drain mast and if you look very close you can see the "Cooper Vane" under the lower empanage to keep the airstairs from being deployed in flight.

TR
 
airplay
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RE: Strange Looking Antenna On B-727?

Mon Jan 14, 2002 1:18 am

Mr. Spaceman,

That antenna on the 172 is an older style marker beacon antenna. Newer antennas are shaped like a little boat, or are flush mounted (no protrusion).

If you look carefully at that picture, you'll see a long wire ADF sense antenna attached to the vertical stab. ADF systems need a sense antenna which is usually a long wire or towel bar, and a loop antenna which is usually a flat oval or tear-drop shaped unit. Of course there are several variations. King (now Honeywell) had a "whip" style sense antenna available at one time but it didn't perform as well as the long wire. They also introduced a combined sense/loop antenna that sort of looked like a VHF "shark's fin" but it was superceded by a large tear drop shaped flatter unit.

There are also some VHF antennas that are bent like that marker beacon antenna. They are specially desinged for mounting on the belly. They do not however have the little tell-tale tuning tap that you see as a wiring connecting to the antenna element.

Also, the talk about the similarity of the 727 towel bar antenna to an ADF sense antenna is somewhat correct. Typically though, an ADF sense antenna only has 3 standoffs. This allows for a center feed. So (usually) 3 standoffs means ADF, more means HF. (The standoffs are the vertical members that support the horizontal element)

The towel bar style ADF sense antennas can be mounted on the top or bottom just as the loop antennas can. You'll see them on top of aircraft that operate on gravel and don't have much ground clearance like a twin otter which use a modified towel-bar that uses 2 end supports with a wire element instead of a rigid one and a small centre-tap.


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Photo © Ian Haskell



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Photo © Alastair T. Gardiner



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Photo © JOE G. WALKER



Other aircraft like the old HS-748 have the sense antennas mounted on the bottom.


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Photo © Rui Dias



This picture shows them at the back of the airplane. Most HS-748s have them at the front but this one used to be a coast-guarder (G-BCDZ) and had a large radar pod on the belly at front. By the way, before anyone even asks, the long wire is the HF antenna.

 Smile

Most modern ADFs use combined loop/sense antennas so you wouldn't normally find long-wire or towel bar ADF antennas on newer airliners.

Like I've said before though, I wouldn't be completely surprised if someone came up with an example that disputed this rule of thumb.

 Smile



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