VHF AM Airband Antenna

[prcguy]

I have the UHF version of the Sirio ground plane and can say its a very clever design. Its more than a fat monopole over ground radials and it has some a fancy matching network down inside the hollow vertical element. From what I've seen Sirio makes very high quality and innovative antennas.
prcguy

I didn't make any claims to it being a 'high performance' antenna. However as prcguy mentioned in post #13:

"Commercial ground plane antennas marketed for VHF air band use a very fat vertical radiator upwards of 1.5" dia or more. These will have a great match across 118-136Mhz."

The vertical radiator on the Sirio is 2-3/8" in diameter. There may be a little more to it than meets the eye. The Vertical radiator is also hollow, and has two screws at the top that allow it to be removed from the rest of the antenna, almost like an upside down can. Internally there is also two brass vertical elements with what appears as some kind of tuning mechanism near the base that bridges across the two, but not as a dead short. I won't pretend to understand the design. All I know is it works great! When I get the time, I'll dismantle it and try to post some pictures of its internal design.

Top view showing the two screws for disassembly:

 

[prcguy]

That's exactly opposite of whats needed for distant aircraft reception and a Discone is one of the best for that. Ever wonder why Discones are fairly common at airports? They cover the entire VHF band with a great match and have a low angle of radiation across that band. The other type of antenna used at airports is a fat vertical dipole with a gain lobe at the horizon and a null overhead. See the bottom of my post 54 in this thread and here is something I posted years ago and just copied:

"Aircraft at high altitude are usually line of sight for quite a distance and can be picked up with a very marginal antenna. For aircraft at extreme distance you want the most gain at the horizon where even at high altitude the aircraft is at or near the horizon.

An antenna having a 20dB or more null straight up is usually not a problem and an aircraft at 35,000ft above you is only about 6.5mi line of sight and even at a 30deg look angle an aircraft at 35,000ft is only about 13mi away, not counting curvature of the earth, etc."

The bottom line is you don't need any gain up in the air for aircraft reception, you want it mostly at the horizon where its needed for distant reception blocked by trees and buildings. At some point you want the gain simply due to distance and path loss if your on a high hill with no blockage in the direction of the aircraft.

I've personally talked quite a distance line of sight and beyond the horizon to aircraft and here is wording from another old post I had to dig up:

"My personal record from an 800ft hill to an aircraft at 34,000ft over water was 338NM as read by the pilot who set my location as a waypoint, which is 388.96 statute miles! Aircraft was a Boeing 737 (20 to 40w?) and a 5w VHF hand held with 1/4 wavelength whip."

The radio on my end was an MBITR feeding a modified wide band Antenna Specialists 1/4 wave whip using a longer whip to cover the VHF aircraft band. That on the roof of my vehicle and at the edge of a high sloping hill had a fairly low angle giving obviously great coverage out past the horizon. If I had an antenna optimized for an upward pattern I would never have logged the impressive distance.

At 389mi we were still communicating and it was getting weak but we could have gone further. The main reason we stopped at 389mi was because it was getting boring.
prcguy

Please stop recommending Discone antennas for air band and satellite reception. The vertical beamwidth of these broadband, no gain antennas put the radiation pattern close and below the horizon - neither of which aircraft or satellites exist.

 

[SDRPlayer]

"At 389mi we were still communicating and it was getting weak but we could have gone further. The main reason we stopped at 389mi was because it was getting boring."

simply awesome!

 

[iMONITOR]

I have the UHF version of the Sirio ground plane and can say its a very clever design. Its more than a fat monopole over ground radials and it has some a fancy matching network down inside the hollow vertical element. From what I've seen Sirio makes very high quality and innovative antennas.
prcguy

Are you refering to the
Sirio GP 430 LB/N 380 - 480 MHz UHF Base Antenna
Sirio GP 430 LB/N 380 - 480 MHz UHF Base Antenna [gp430lb_n] - $104.00 : RMItaly.US, The RM Italy Store!


Are you using this for amateur radio, or something more specialized?

I bought one of these for monitoring Federal Disaster frequencies. Especially USAR (Urban Search and Rescue), and FEMA. Some are encrypted, most are not. Some digital, some analog. Might be interesting to try on FRS/GMRS. I got it too late in the year to get it up on the roof.

 

[prcguy]

I bought two of those cheap to play with but have not used them yet except to take apart and study the construction.
prcguy

Are you refering to the
Sirio GP 430 LB/N 380 - 480 MHz UHF Base Antenna
Sirio GP 430 LB/N 380 - 480 MHz UHF Base Antenna [gp430lb_n] - $104.00 : RMItaly.US, The RM Italy Store!


Are you using this for amateur radio, or something more specialized?

I bought one of these for monitoring Federal Disaster frequencies. Especially USAR (Urban Search and Rescue), and FEMA. Some are encrypted, most are not. Some digital, some analog. Might be interesting to try on FRS/GMRS. I got it too late in the year to get it up on the roof.

 

[paulears]

I too am amazed by those who denigrate discones when conical design antennas in operation at almost every aviation centre. Its quite rare to see any ground plane types.

 

[rocky28965]

I bought two of those cheap to play with but have not used them yet except to take apart and study the construction.
prcguy

So what did you find inside.
Was it something that could be made up by an average kiwi with number 8 wire?

 

[prcguy]

No, I don't think even a high end Kiwi could figure out the matching network without a bunch of test equipment. I'll have to post a picture when I get home from travel.
prcguy

So what did you find inside.
Was it something that could be made up by an average kiwi with number 8 wire?

 

[paulears]

If they are quarter waves, then there should not be a matching network - 50 Ohm design matches to 50 Ohm cable and receiver perfectly. I'm interested in the claim for DC path for lightning protection - maybe this is what the matching network is - however, if the vertical is a DC grounded component, I'm not sure how it would function? It would have to be something other than a 50Ohm component and then have some kind of stub or gamma match, which makes no real sense. I've never seen a DC bonded discone, or non-folded dipole? Love to see the picture.

 

[dlwtrunked]

Please stop recommending Discone antennas for air band and satellite reception. The vertical beamwidth of these broadband, no gain antennas put the radiation pattern close and below the horizon - neither of which aircraft or satellites exist.

Please note that many of us require wideband omnidirectional antennas and use what the military and FAA do for broadband--a discone--mine came from FAA surplus. Just because the optimal gain may not be in the right place does not mean they are not useful and no omnidirectional antenna will have gain. If you are trying to only hear a distant airport in one direction, then the discone is not a good choice.

 

[Ubbe]

no omnidirectional antenna will have gain.

Only 1/4 wave GP have no gain. All other types of omnidirectional verticals have gain. There are verticals for the amateur bands that have 10dB gain.

/Ubbe

 

[paulears]

I think he's probably referring to the concept of the isotropic source - the true omni directional antenna that doesn't exist.

I can't believe he meant what he said - I think it's all the talk about polar patterns and radiation angles - because changing those is the only way to achieve gain - as in the setting one off against the other.

 

[dlwtrunked]

Only 1/4 wave GP have no gain. All other types of omnidirectional verticals have gain. There are verticals for the amateur bands that have 10dB gain.

/Ubbe

Yes, I meant to say broadband and omnidirectional.

 

[prcguy]

There is that to some extent, you can get stacked dipoles for the 118-136 or 225-400Mhz range with upwards of 6dB gain. I have a four stack for 225-400Mhz that 11ft long and works great.
prcguy

Yes, I meant to say broadband and omnidirectional.

 

[dlwtrunked]

There is that to some extent, you can get stacked dipoles for the 118-136 or 225-400Mhz range with upwards of 6dB gain. I have a four stack for 225-400Mhz that 11ft long and works great.
prcguy

One of the oddest antennas that I have is antenna of a pair stacked discones for 225-400 MHz which came from Italy. Large, heavy, and awkward. It is not clear to me that one should expect significantly better performance from such (which is probably why I have never seen it elsewhere). never HAD A PLACE TO MOUNT IT.

 

[prcguy]

Aah, so you bought one of those things! They should be of very good quality, but at the very best you will get 3dB gain at the optimum frequency of the spacing, and less everywhere else.

I could have sold you my Chu & Associates 6dB gain 225-400MHz 11ft omni for less.
prcguy

One of the oddest antennas that I have is antenna of a pair stacked discones for 225-400 MHz which came from Italy. Large, heavy, and awkward. It is not clear to me that one should expect significantly better performance from such (which is probably why I have never seen it elsewhere). never HAD A PLACE TO MOUNT IT.

 

[Ubbe]

There are 8 stacked dipoles for 410MHz-470MHz that have 13dBi gain.
RigPix Database - Antennas - Aerial OY AV1914-8

/Ubbe

 

[paulears]

Sorry - but I'm missing the point here? If you go up to 16 elements you get another 3dB - but it's also the law of diminishing returns, as the losses in the phasing harnesses start to be significant, so in real life - 4 stacks tend to be far more common, and of course these things are directional too - as in your image, which have a gentle cardioid polar pattern in the H plane, which you can restore to nearly omni if you stagger them around the pole in 90 degree jumps, with a corresponding drop in overall gain.

They also concentrate their radiation closer to the horizon - which is what the topic was about. Upwards is pretty much pointless, horizontal has an above and a below component which wastes energy from an aviation perspective - so what you ideally want is a radiation pattern that gently compensates for free space losses as the source gets further away, and lower in angle. Keeping the gain distribution tracking the increasing angle and distance works best.

 

[prcguy]

That's not a broad band antenna as I was discussing. When you get omni directional with gain close to an octave or BW more like the 225-400Mhz band, that takes design smarts well beyond regular mortals.

And the losses adding up in the phasing harness of stacked dipoles is nothing to worry about. They are fed from the center of a corporate phasing harness and on a 16 bay UHF array you might have 10ft lengths of low loss coax with only a fraction of a dB loss in the harness.
prcguy

There are 8 stacked dipoles for 410MHz-470MHz that have 13dBi gain.
RigPix Database - Antennas - Aerial OY AV1914-8

/Ubbe

 

[paulears]

Really? Oh well, perhaps the gain figures Jaybeam, before they became Amphenol used to publish clearly showed that doubling the number of driven elements didn't double the gain - close, but unfair to say they're lossless. In fact most ops the faults we used top swap these out for were caused by degeneration of the harness assemblies, not the antennas themselves. A 6dBd theoretical gain losing just over 1dB in the harness. I realise 1dB is very little - but to say they are lossless is just not right.