Moble and Base Antennas for Air

[n2dpsky]

I've been pushing my little Uniden Bearcat BC125AT scanner and its Diamond RH77CA antenna to their limits for aircraft monitoring, but I want more. There aren't a lot of air band antenna options out there, so I sought the advice of my local Ham Radio Outlet. They told me I was heading the right direction, but didn't have options for me.

First, I'm looking for a magnetic mount mobile antenna. I'm pretty sure I want a 5/8 wave. Here in Southern California commercial FM transmissions are pretty significant, so I'm erring on the side of caution. My first question is this. Is 1/4 wave good enough or will I see the benefit of 5/8 wave for receive only? Again, I'm trying to isolate air band from FM.

Second, I've never built an antenna and am not really well versed in antenna theory, but it seems to me that a mag mount and a whip cut to length (58 1/8" for 5/8 wave or 23 1/4 for 1/4 wave), centered on 127MHz would work. Is there more to it than that?

If I'm successful for the mobile antenna, I'll try my hand at a base ground-plane antenna.

Thanks in advance.

 

[prcguy]

A typical 5/8 mobile antenna like a Larsen, A/S and similar are fairly narrow band and after a few MHZ their performance will drop off noticeably from where they are tuned. A simple 1/4 wave whip will be more consistent over the entire VHF air band and there are some wide band versions you can find on Ebay that should cover the entire 118-136Mhz range no problem.

For air band base antennas, PM me if your in So Cal.
prcguy

I've been pushing my little Uniden Bearcat BC125AT scanner and its Diamond RH77CA antenna to their limits for aircraft monitoring, but I want more. There aren't a lot of air band antenna options out there, so I sought the advice of my local Ham Radio Outlet. They told me I was heading the right direction, but didn't have options for me.

First, I'm looking for a magnetic mount mobile antenna. I'm pretty sure I want a 5/8 wave. Here in Southern California commercial FM transmissions are pretty significant, so I'm erring on the side of caution. My first question is this. Is 1/4 wave good enough or will I see the benefit of 5/8 wave for receive only? Again, I'm trying to isolate air band from FM.

Second, I've never built an antenna and am not really well versed in antenna theory, but it seems to me that a mag mount and a whip cut to length (58 1/8" for 5/8 wave or 23 1/4 for 1/4 wave), centered on 127MHz would work. Is there more to it than that?

If I'm successful for the mobile antenna, I'll try my hand at a base ground-plane antenna.

Thanks in advance.

 

[toastycookies]

Have you checked out options from DPD?

DPD Productions - VHF Air/Aviation Band, FBO Base, PCL, AWOS, ACARS & ADS-B Antennas

 

[n2dpsky]

Have you checked out options from DPD?

DPD Productions - VHF Air/Aviation Band, FBO Base, PCL, AWOS, ACARS & ADS-B Antennas

In fact that the was very antenna under consideration, but it was a little pricey for what it was.

The more I explored the subject, the more it seemed I was overthinking it. For receive only, it did not appear to be as critical. I ended up ordering a mag mount NMO base and two whips, one Larsen NMOQ88C field tunable whip and one PCTEL ASPR795.

One whip was tuned more closely to air band and one was broader extended up to mil air and 70cm HAM when needed. The whips themselves were less than $20 each, so it wasn't a hard decision to see how they work. They should be here next week, so I'll report back.

 

[wyShack]

A 5/8 wave antenna has 'gain' by increasing signal at the horizon. A 1/4 wave will actually be better if what you are listening to is more than about 30 degrees off the horizon -like airplanes overhead. Normally they are close if they are off the horizon so it is a trade off. I have found the 1/4 wave to be more broad banded.

 

[n2dpsky]

A 5/8 wave antenna has 'gain' by increasing signal at the horizon. A 1/4 wave will actually be better if what you are listening to is more than about 30 degrees off the horizon -like airplanes overhead. Normally they are close if they are off the horizon so it is a trade off. I have found the 1/4 wave to be more broad banded.

I understood the 1/4 was broader than the 5/8, but thanks for info about the angle off the horizon. That was new information. One question though. The ground based transmission from ATC are the tough ones and the aircraft was easier due to the better line of sight. so wouldn't 5/8 help in the area most needed or would the decrease in performance in the air offset the horizon level gain?

Thanks,
Chris M.

 

[prcguy]

There is a lot of misconception about aircraft reception and the need to have an antenna that points upward. Usually the opposite is true because when the aircraft is at higher angles like 30deg, 60deg or straight above you they are usually line of sight and really not very far away so you can usually pick them up with little to no antenna.

Its when aircraft are at an extreme distance at the horizon and the signals are going through lots of buildings, trees, etc in the way, or you are trying to pick up ground stations when antenna low angle and gain are your friend.
prcguy

I understood the 1/4 was broader than the 5/8, but thanks for info about the angle off the horizon. That was new information. One question though. The ground based transmission from ATC are the tough ones and the aircraft was easier due to the better line of sight. so wouldn't 5/8 help in the area most needed or would the decrease in performance in the air offset the horizon level gain?

Thanks,
Chris M.

 

[ko6jw_2]

A 5/8 wave antenna is harder to home brew because it's not just the whip length, it's matching to 50 ohms. This requires a coil at the base of the antenna.

Commercial mobile air band antennas are made for use on aircraft. I recently worked on the avionics in a glider where the antenna was built into the tail of the aircraft. It was a half-wave vertical. Obviously, wind resistance is a big factor.

You could easily build a j-pole for the air band. However, the j-pole is not very wide band when you are interested in 118-136Mhz. A discone is a better bet and also will cover military air frequencies.

Gain is less important than bandwidth. Hearing ground stations is more difficult than aircraft as has been said in other answers. That is the nature of air monitoring.

 

[K2RNI]

There is a lot of misconception about aircraft reception and the need to have an antenna that points upward. Usually the opposite is true because when the aircraft is at higher angles like 30deg, 60deg or straight above you they are usually line of sight and really not very far away so you can usually pick them up with little to no antenna.

Its when aircraft are at an extreme distance at the horizon and the signals are going through lots of buildings, trees, etc in the way, or you are trying to pick up ground stations when antenna low angle and gain are your friend.
prcguy

Words right out of my mouth PRC. I too use a 5/8 for Aviation and noticed the same thing. The ones coming down at the higher angles are always close and strong enough to not be an issue and anything further away or lower like ground and AWOS hit the sweet spots on the side just nicely. I can hear our local ASOS over 7 miles of house and get a good copy on both sides of the conversation from Las Vegas ATC 90 or so miles away.

It's a pretty good setup especially considering I'm only using just RG8X at 40 feet.

 

[n2dpsky]

Base VHF Ground Plane Antenna

Well, I promised I'd report back once my antenna was built and tested. I'm happy to say, it works pretty well. Below is a link to the other thread where I detailed the build.

Highlights: It's an SO-259 mount built onto an aluminum project box. I used a PL-259 to "F" female connector to join it to the RG-6 coax and an 18 inch "F" female to BNC male pigtail on the other end to connect to the receiver. I used #12 solid copper wire from Home Depot (.18/ft) for vertical/radials The vertical and radials are both cut for 1/4 wave at 127 MHz. The radials I soldered to electrical connectors to make screwing to the box and updating later a bit easier. Right now the whole thing is clamped to a PVC stand I made for testing. Once I'm happy, I'll mount it in my attic.

Upgrades: I've already planned to change the radials to a larger diameter to see if I can get more bandwidth out of it, perhaps #10 or #8 wire. Signal is not as strong at the lower end and the airpot I monitor has a tower freq nearby. While I'm at it I'll add 5% to the length, although my research suggests the added length may not be as much benefit as conventional wisdom suggests. Aside from the that, I'm pretty pleased for my first antenna.

Cost: All told, I spend $40 for parts and supplies. The most expensive part of the 18 inch "F" to BNC pigtail ($10.99), which was necessary if I wanted my handheld scanner to stand on it's own. The project box was $7.59 and not necessary, but made the project look nicer. I had to buy the right screws and connectors, so that added a couple dollars, but you could still build this for $24 without the pigtail or the box. More importantly, I learned something and I had a really good time researching it and putting it together.

http://forums.radioreference.com/sc...4691-antenna-tuning-air-band.html#post2591896

Thanks to everyone for suggesting I try this. I was intimidated at first, but it began making more and more sense as I went along.

Regards,
Chris M.

 

[AnikF1R]

I've been looking for information on building a 5/8 wave antenna for the air band, but without much luck. I'd like to pick up ground services (ASOS, etc.). It's the coil that I can't figure out. Short of buying an antenna analyzer, how do you know things like coil diameter, # of turns, where to tap?

Thanks

 

[n2dpsky]

I've been looking for information on building a 5/8 wave antenna for the air band, but without much luck. I'd like to pick up ground services (ASOS, etc.). It's the coil that I can't figure out. Short of buying an antenna analyzer, how do you know things like coil diameter, # of turns, where to tap?

Thanks

Mine was a simple 1/4 ground plane antenna, but it could be build in 5/8 as well just by lengthening the vertical and radials. However, it sounds like yours may be more complex than mine. However, I can send you some photos and a list of what I did if you'd like.

I'm now looking into antennas to build that have gain. If you're looking at ground comms of a particular airport, a directional beam antenna like a tape measure yagi might do it. I need something omni-directional with 2 to 3 db of gain, like a collinear vertical.

Chris M.

 

[AnikF1R]

I would just lengthen the main radiator, but I want my radio to see a 50 ohm impedance, which will require a tapped coil, or something like that.

A yagi sounds like a good choice. They're fairly large at these frequencies, however, and I've got to put one on top of the roof vertically.

 

[n2dpsky]

I would just lengthen the main radiator, but I want my radio to see a 50 ohm impedance, which will require a tapped coil, or something like that.

A yagi sounds like a good choice. They're fairly large at these frequencies, however, and I've got to put one on top of the roof vertically.

I'm certainly no expert. The 1/4 wave ground plane I built (my first) should have a 50 ohm impedance with the radials at 45 degrees (although I'm using 75 ohm RG-6 as a feedline). It doesn't seem to matter for receive only, which is all I'm doing. That can be changed though. It just doesn't have any gain and its not directional. I scan about 45 air frequencies from my home so a Yagi wouldn't work for me. I thought of building a 5/8 variant of the same ground plane antenna to see of I get more signal from the horizon, but then I loose bandwidth. It's cheap so I may do it anyway just for the learning experience. I've also looked at building a collinear vertical do get a few db of gain. That would be ideal for my situation.

Chris M.

 

[AZ645]

Would an actual aircraft antenna such as a DMC 50-17 work for air band reception? Has anyone actually used one for reception?