strange antenna

[k2az42]

Can anyone give me a technical explanation of the antenna in the attached picture?

John - K2AZ

 

[fineshot1]

Not sure what you want to know & I can not get too technical on this one but they are common on aircraft and there are models for the vhf & uhf air bands. Not sure which one this is.

 

[k2az42]

This antenna is on an old VC130 Air Force 1. We. (Air and Space Museam volunteers) get a lot of questions about what it is. I believe it was used for TV reception. It is circa 1960's 70's. From its construction and size I would guess it is broadband vhf and might have some loop and/or log periodic characteristics. I would like to provide a credible response to visitors, some of which have a basic background in aeronautis and radio.

thanks, John

 

[SAR923]

Is it a VC-130 or RC-130? I doubt it was used for TV reception, more likely UHF airband, but RC-130's had tons of spook antennas that could monitor almost any frequency. A VC-130 was used for VIP transport and watching off the air TV wouldn't be easy at 20,000 feet.

 

[k2az42]

The aircraft is a VC-137C, SAM970 (a modified Boeing 707-153). It was the first jet aircraft used as Airforce One by Eiswnhower through Nixon. The TV thing sounded fishy to me too that's why I asked the question but the docents at Boeing claim it was for television reception by the press corps.

 

[prcguy]

Is there two of these antennas, one on each side of the aircraft? Comparing the antenna to the rivet size it looks to be 6 or 8 inches long and maybe 6inches tall. It also resembles a folded monopole, which is a grounded broad band quarterwave antenna. My guess is its one of two glide path antennas for navigation that operate around 330MHz.
prcguy

 

[SAR923]

Ah, OK, a VC-137 is a completely different animal than a VC-130. I agree with pcrguy, there should be another antenna on the other side that's the same or similar and it was used for glide path navigation.

 

[k2az42]

Proguy's explanation sounds on target to me. It is a little larger though, maybe 10 or 12 inches lomg by 8 tall and a monopole of some kind. I believe there is a matching antenna on the other side of the aircraft. I was confused about one side apparently being grounded. My antenna knowledge is limited. I felt foolish telling people it was for television reception. That just doesn't sound right to me.

Thanks guys

John

 

[Hooligan]

I've got some old tech manuals for the VC-137Bs & VC-137Cs (& was aboard 58-6972 while it was in service), and I know they have a diagram pointing out each & every antenna. Gimme a little time, but I'll look it up for you. Please do tell me exactly where on the fuselage the antenna is.


Tim

 

[Hooligan]

The TV thing sounded fishy to me too that's why I asked the question but the docents at Boeing claim it was for television reception by the press corps.

Press Corps sure didn't get an opportunity to watch TV back in their compartment -- they were only supposed to know what the DV/media handlers wanted them to know while aboard the plane!

They did experiment with TV reception for the DV, though.

 

[kb2vxa]

Never mind it's size, that only determines it's resonant frequency. The question was what type of antenna is it and the answer is a grounded unipole. Look at it as one half of a folded dipole with the far end grounded and the near end is the feed point. It presents approximately 400 ohms impedance so it requires a tuning network to bring it down to 50 ohms if that is required by the equipment. The distance from the mounting surface to the farthest point from it is just shy of one quarter wave.

Actually it's the door handle of a fancy sports car but don't tell anybody you're goofing on them.

 

[k2az42]

To Hooligan,

The antenna is below wing level, behind the wing, about halfway between th back of the wing and the rear passanger door. Alan Rudge refers to a "Bull-Ring Glidepath Antenna" in his book but don't know if that has any thing to do with any of this other than that's what I would invision it to look like.

John

 

[prcguy]

I believe a folded unipole has several wire elements. The antenna in question is a folded and grounded 1/4 wave monopole and the impedance of these are around 50 ohms. The antenna is low impedance (50 ohm) at the feed point, several thousand ohms at the far end which is 1/4 wavelength out and when it bends back to the feedpoint its low impedance again and it can be grounded. Besides providing a grounded element for static discharge the folded monopole has wider BW over a simple 1/4 wave ground plane.
prcguy

Never mind it's size, that only determines it's resonant frequency. The question was what type of antenna is it and the answer is a grounded unipole. Look at it as one half of a folded dipole with the far end grounded and the near end is the feed point. It presents approximately 400 ohms impedance so it requires a tuning network to bring it down to 50 ohms if that is required by the equipment. The distance from the mounting surface to the farthest point from it is just shy of one quarter wave.

Actually it's the door handle of a fancy sports car but don't tell anybody you're goofing on them.

 

[Hooligan]

To Hooligan,

The antenna is below wing level, behind the wing, about halfway between th back of the wing and the rear passanger door. Alan Rudge refers to a "Bull-Ring Glidepath Antenna" in his book but don't know if that has any thing to do with any of this other than that's what I would invision it to look like.

John

Sorry for the delay. Turns out the "USAF Series Aircraft VC-137C Communications and Electronics Inflight Operations Manual" has been just a few feet behind this computer instead of boxed up in another room as I thought it'd be!


Anyway, I'm looking at an antenna locations diagram, and based on the location info you provided, as other people pointed out, this would be a TV antenna, and there should be an identical one on the other side of the fuselage. It should be a few feet closer to the rear passenger door than to the trailing edge of the wing where it attaches to the fuselage.

The "Guide Slope Antenna" is located on the nose of the plane.

Note that this data is for the C-137C aircraft. Your bird was a C-137B, but some of the antenna location data is going to be identical. I think I do still have some C-137B comms-electronics manuals, so I'll check that source once i dig one up.


Tim

 

[chrismol1]

Sorry for the delay. Turns out the "USAF Series Aircraft VC-137C Communications and Electronics Inflight Operations Manual" has been just a few feet behind this computer instead of boxed up in another room as I thought it'd be!


Anyway, I'm looking at an antenna locations diagram, and based on the location info you provided, as other people pointed out, this would be a TV antenna, and there should be an identical one on the other side of the fuselage. It should be a few feet closer to the rear passenger door than to the trailing edge of the wing where it attaches to the fuselage.

The "Guide Slope Antenna" is located on the nose of the plane.

Note that this data is for the C-137C aircraft. Your bird was a C-137B, but some of the antenna location data is going to be identical. I think I do still have some C-137B comms-electronics manuals, so I'll check that source once i dig one up.


Tim

nuthin like useful aircraft manuals handie lol, where did you get those from?

 

[k2az42]

Thanks for everyones help and comments on this one, especially Tim. I guess it is official. To my surprise, they actually are television antennas - so none of the visitors at the Boeing Air and Space Museum were misinformed. Now I have to verify the hearsay about the ECM pods mounted behind the jet engines spitting out metallic chaff through their mesh ports, but that is a picture and question for a different forum.

Best regards to all, John - K2AZ, Seattle WA

 

[Hooligan]

Thanks for everyones help and comments on this one, especially Tim. I guess it is official. To my surprise, they actually are television antennas - so none of the visitors at the Boeing Air and Space Museum were misinformed. Now I have to verify the hearsay about the ECM pods mounted behind the jet engines spitting out metallic chaff through their mesh ports, but that is a picture and question for a different forum.

Best regards to all, John - K2AZ, Seattle WA

Ah, as for the countermeasure pods, that sort of stuff is still kinda sensitive these days, but you can tell people that they were installed circa early 1980s, and at least when it was actually supposed to be highly classified, it was just referred to as "the system." After some of the 89th Wing aircraft got them, I know NASA put them on their 747 space shuttle transportation aircraft. You might be able to check with NASA to see if their employment was/is unclassified. Aviation Week & Space Technology magazine had some minor articles about the system. Being located behind the engine nacelles gives a clue as to what sort of surface to air threat they were countermeasures for. The System was just designed for the small variety of man-portable surface to air missiles, so the system was turned off when the aircraft was above a couple thousand feet. If I recall correctly (I'll check the same manual), controls for it were at the flight engineer's station.


I got these manuals from friends who were involved in Presidential communications.


Tim

 

[scanpro12]

weird lol.