Air Band Antenna question

[Lexxx]

Hi

Looking for antenna help.

I have a scanner that is used exclusively for the reception of several air traffic frequencies (the 108-137 MHz range)

I take the external speaker signal and run it into the sound input on a computer, and then out through the internet. It works well enough, but I notice the sound quality from other similar sites can be slghtly better.

I'm quite close to the airport so distance is not the issue. The scanner (Uniden BC350C) just has the telescoping antenna it came with, and sits on a shelf on the top floor of my house. As mentioned it does a decent job, however, I'm looking for the best I can manage on a reasonable budget. I'm hoping to put some kind of antenna in the attic (want to aviod a trip to the roof if at all possible).

Do you think I'm going to get a noticeable improvement, and what sort of antenna would you recommend for this frequency band?

Any help appreciated.

 

[N_Jay]

Hi

Looking for antenna help.

I have a scanner that is used exclusively for the reception of several air traffic frequencies (the 108-137 MHz range)

I take the external speaker signal and run it into the sound input on a computer, and then out through the internet. It works well enough, but I notice the sound quality from other similar sites can be slghtly better.

I'm quite close to the airport so distance is not the issue. The scanner (Uniden BC350C) just has the telescoping antenna it came with, and sits on a shelf on the top floor of my house. As mentioned it does a decent job, however, I'm looking for the best I can manage on a reasonable budget. I'm hoping to put some kind of antenna in the attic (want to aviod a trip to the roof if at all possible).

Do you think I'm going to get a noticeable improvement, and what sort of antenna would you recommend for this frequency band?

Any help appreciated.

I would guess you want both sides of the conversation.
I would use a low gain omni antenna cut for the center of the band.

I would think a simple wire dipole might work well (and be cheap).
Use decent feed line and keep it from getting too long.

 

[LarrySC]

The word is "capture area" the more antenna you have in the air the more signal you capture. The secret however is the antenna must be cut for the frequency range you need. With aircraft this is the only time this rule does not allways apply. Aircraft signals from overhead are neither Vert or Horz. Thats why flat top discones are used for aircraft. SO just install a simple flat top discone cut for 118-137 or above. Use a good grade coax and connectors. Good Luck, Larry

 

[N_Jay]

The word is "capture area" the more antenna you have in the air the more signal you capture. The secret however is the antenna must be cut for the frequency range you need. With aircraft this is the only time this rule does not allways apply. Aircraft signals from overhead are neither Vert or Horz. Thats why flat top discones are used for aircraft. SO just install a simple flat top discone cut for 118-137 or above. Use a good grade coax and connectors. Good Luck, Larry

Well a "discone" antenna, is a Disc on top of a Cone.

What do you mean by a flat top?

Discones are good for wide bandwidths. You realy don't need that just for VHF aircraft band.

As for polerazation, yes Aircraft signals may be a bit off vertical, but you don't lose much till you are way out (greater than 60 deg) so a vertical should work fine.

You can experiment with a dipole with multiple radials on both poles separated by 60 deg (30 off vertical) and see if it helps.

 

[hdcoley2]

Air Band Antenna Question

Discone antennas are the best for aviation for 2 reasons, they are broadband and another very good benefit is that they have a radiation pattern that is skyward.

This is why you have to be so close to an airport facility to receive the ground station well. The discone antenna does not propagate much out to the horizon, instead the pattern is elevated skyward in an omnidirectional pattern.

On a Smith chart the pattern resembles a pear standing on the small end. They are primarilly vertically polarized.

Any vertical antenna will work well for aviation reception. If it is not a discone, it should be a 1/4 wave vertical and not a vertical with gain over a 1/4 wave vertical antenna. 1/2 wave,5/8 wave, 3/4 wave and colinear antennas have gain over a 1/4 wave antenna because their radiation patterns are flatter and therefore extend farther out to the horizon than up or down. A 1/4 wave antenna has a pattern that is closer to a ball, or one than is pushed down a bit, so it has a great deal of skyward pattern. If it is cut for the center of the band, it will work well enough on VHF aircarft. It does not have to be a great match on the low and high end of the band for good reception. Discones are more broadbanded at the expense of losing effective radiation power on the entire band. Antennas are always a compromise, especially if covering a broad range of frequencies. Your first choice would be a Discone and then a 1/4 wave vertical. If monitoring one single frequency, a 1/4 wave cut for that frequency would do as well as a discone.

 

[loumaag]

Re: Air Band Antyenna Question

Oh my, there is so much wrong with this I don't know where to start...but here goes:

Discone antennas are the best for aviation for 2 reasons, they are broadband and another very good benefit is that they have a radiation pattern that is skyward.

True and false. Yes they are broadband and hence popular as multiband antennas. No, they do not have a "skyward" radiation pattern. As a matter of fact the radiation angle from a discone is quite low and that is another well known plus for this antenna.

This is why you have to be so close to an airport facility to receive the ground station well. The discone antenna does not propagate much out to the horizon, instead the pattern is elevated skyward in an omnidirectional pattern.

Not quite, see my resopnse to fhe first part above.

On a Smith chart the pattern resembles a pear standing on the small end. They are primarilly vertically polarized.

Smith charts are not used to display or calculate radiation patterns from an antenna. They are a rather sophisticated tool used to solve transmission line and impedance matching problems.

Any vertical antenna will work well for aviation reception. If it is not a discone, it should be a 1/4 wave vertical and not a vertical with gain over a 1/4 wave vertical antenna. 1/2 wave,5/8 wave, 3/4 wave and colinear antennas have gain over a 1/4 wave antenna because their radiation patterns are flatter and therefore extend farther out to the horizon than up or down. A 1/4 wave antenna has a pattern that is closer to a ball, or one than is pushed down a bit, so it has a great deal of skyward pattern. If it is cut for the center of the band, it will work well enough on VHF aircarft. It does not have to be a great match on the low and high end of the band for good reception. Discones are more broadbanded at the expense of losing effective radiation power on the entire band. Antennas are always a compromise, especially if covering a broad range of frequencies. Your first choice would be a Discone and then a 1/4 wave vertical. If monitoring one single frequency, a 1/4 wave cut for that frequency would do as well as a disicne.

Uh-huh. :?

 

[N_Jay]

I think I am contagous! :twisted: :twisted: :twisted:

Actually, ity does seem afer a bit of searching that either a discone is a popular antenna for commercial use in teh airnautical band. (Can't be too bad).

I would still probabaly build a dipole with "bowtie" like poles.

Or something that looks like a dipole using cones.

 

[K2KOH]

Depending upon how far away you are from the airport, I might invest in a VHF directional antenna, and retune it for the 118-136 MHz band. It can be done if you lengthen the elements a bit.

 

[KD4UXQ]

Airband antenna

loumaag,

Why are Discones used for aviation? Is it primarily their broadband nature? It would seem that a 1/4 wave vertical dipole cut for the transmitting frequency would do better.

I did some searching on the internet and can't really find a reason why they are used in aviation ground sites. But they certainly are. You see them at every airport it seems.

I've seen Smith charts of antennas that looked like a radiation pattern. Is it actually showing the impedance of the antenna, which resembles it's pattern?

 

[N_Jay]

Re: Airband antenna

loumaag,

I've seen Smith charts of antennas that looked like a radiation pattern. Is it actually showing the impedance of the antenna, which resembles it's pattern?

OK, a Smith Chart is used to calculate many factors like impeadance in a trasnmission line system, HOWEVER, I have never seen a "Smith Chart" of an antenna.

What you are probabaly seeing are polar graphs of the radiation paterns of an antenna in either the vertical or horizontal plane.

 

[KD4UXQ]

Re: Airband antenna

loumaag,

I've seen Smith charts of antennas that looked like a radiation pattern. Is it actually showing the impedance of the antenna, which resembles it's pattern?

OK, a Smith Chart is used to calculate many factors like impeadance in a trasnmission line system, HOWEVER, I have never seen a "Smith Chart" of an antenna.

What you are probabaly seeing are polar graphs of the radiation paterns of an antenna in either the vertical or horizontal plane.

They were labled Smith Charts, but I can't find an example now. Antway, any ideas on the reason for Discones used in aviation?

 

[loumaag]

Re: Airband antenna

Why are Discones used for aviation? Is it primarily their broadband nature? It would seem that a 1/4 wave vertical dipole cut for the transmitting frequency would do better.

Well, since I am not particularly into aviation I can only make a guess. The aviation frequencies cover a pretty wide swath of spectrum. For example, take a look at the Minneapolis Air Route Traffic Control Center here on the site. I have designed and built antennas for quite a number of years, but I don't know how to build a 1/4 ground plane that is going to cover 20 MHz wide section of bandwidth at VHF much less 200 MHz! Now, if you want to look at the small rural type airports which may only have a single radio on one or two Unicom frequencies, you will find they use ground plane antennas, or at least everyone I have ever seen.

As N_Jay points out, you have no doubt seen an antenna pattern sheet, which although is a circle, does not really look like a Smith Chart.

 

[KD4UXQ]

Re: Airband antenna

Why are Discones used for aviation? Is it primarily their broadband nature? It would seem that a 1/4 wave vertical dipole cut for the transmitting frequency would do better.

Well, since I am not particularly into aviation I can only make a guess. The aviation frequencies cover a pretty wide swath of spectrum. For example, take a look at the Minneapolis Air Route Traffic Control Center here on the site. I have designed and built antennas for quite a number of years, but I don't know how to build a 1/4 ground plane that is going to cover 20 MHz wide section of bandwidth at VHF much less 200 MHz! Now, if you want to look at the small rural type airports which may only have a single radio on one or two Unicom frequencies, you will find they use ground plane antennas, or at least everyone I have ever seen.

As N_Jay points out, you have no doubt seen an antenna pattern sheet, which although is a circle, does not really look like a Smith Chart.

Broadband was the only reason I could come up with. Also you are right, I have seen small airports use a vertical. Maybe at the commercial airports they use one antenna for many receivers, or they are prepared to be able to change frequencies at some point even thought they rarely do.

It's easy to see why the military uses a Discone on 225-400 needing a broadband there, and probably changing frequencies often.

 

[N_Jay]

Re: Airband antenna

loumaag,

Why are Discones used for aviation? Is it primarily their broadband nature? It would seem that a 1/4 wave vertical dipole cut for the transmitting frequency would do better.

It could go back to the days when transmitters were much more sensitive to SWR.

The aviation frequencies are fairly wide spread, and a dipole or ground plane would not be as good a match across the band.

Also, things in aviation change VERY slowely, so the historical need might be driving the common use.

 

[Lexxx]

Just wanted to thank all for their responses.

I ended up building what I believe is referred to as a "quarter wave" antenna cut to the main frequency I'm interested in (126.0).

I mounted it my attic with excellent results (www.ykf.ca)

Thanks to all

 

[OpSec]

Looks good.

It will work very well for your needs.