Trying to monitor distant airfield from home...

[mlfd612]

Hi all,

I work on Fort Benning, Ga and want to monitor the airfield better (KLSF/Lawson AAF) from home.
I am not new to scanning and basic antenna theory, but wanted some good outside advice from the wealth pool of it here.
I currently have a scantenna at 750' (home elevtion is 730' plus a 20' mast), and the transmitting antenna is roughly 363' in elevation at 24.5 miles line-of-sight distance. There are no terrain obstacles between the two points, so essentially I am a bit higher than the transmitting station.
I can currently hear the more elevated aircraft transmissions fairly well, as expected.
My goal is to try to hear the tower transmissions on 121.7/119.05 AM, as well as to better hear the aircraft when they are lower in the traffic pattern. I will be using different scanners, but currently it is on an old base station of a Uniden BC898T. Probably would monitor some on the same antenna but downcoupled to RTL-SDR's sometimes.
My cable run from the antenna is fairly long at 90' of RG-6 line, but has served fairly well so far.

I'm thinking some kind of directional antenna? Question is two fold then, I suppose:
1. Discone etc. vs. directional or amplified antenna, and...
2. Would it really be that much of an improvement, or is it an unreachable pipe dream at that distance?

Thanks all!

 

[alcahuete]

One of the towers I can receive from my house is 95' above me (not counting my tower height or theirs), line of sight with really no obstacles, at 15.5 miles. In fact, I can see the tower from my house. I have no problem receiving that with just a handheld, both VHF and UHF. Another tower is 20 miles away, 300' higher in elevation with terrain/obstruction in between, and that requires an external antenna to be heard, also VHF and UHF.

So it definitely isn't unreachable at that distance, but at the least, you are going to need an external antenna. Directional vs. omni is anyone's guess. I'm a big fan of directional antennas, but when you're also dealing with aircraft and not just the tower, directional antennas can be a hinderance, because not every aircraft you might want to hear is the same direction as the airport.

 

[mlfd612]

One of the towers I can receive from my house is 95' above me (not counting my tower height or theirs), line of sight with really no obstacles, at 15.5 miles. In fact, I can see the tower from my house. I have no problem receiving that with just a handheld, both VHF and UHF. Another tower is 20 miles away, 300' higher in elevation with terrain/obstruction in between, and that requires an external antenna to be heard, also VHF and UHF.

So it definitely isn't unreachable at that distance, but at the least, you are going to need an external antenna. Directional vs. omni is anyone's guess. I'm a big fan of directional antennas, but when you're also dealing with aircraft and not just the tower, directional antennas can be a hinderance, because not every aircraft you might want to hear is the same direction as the airport.

Thanks Alcahuete.

I would be monitoring this airfield only, both tower and traffic pattern. I have another receiver and antenna for wider milair monitoring. This would be specifically set-up for KLSF.
Thanks!

 

[alcahuete]

It's definitely worth a try. Can't guarantee one way or another of course, as 25 miles is a pretty long haul, but if it's line of sight with no obstructions, it is definitely worth a shot. Can you see the rotating beacon or airport environment at all from your house?

 

[majoco]

When people say "directional" they think of one of those multi-element yagis with a beamwidth of 20degrees or so - but a simple dipole plus a director will concentrate the reception into a cardiod (heart-shaped) pattern - so you get some more gain from the dipole and even more with the added reflector compared to a discone.

 

[iMONITOR]

Sirio WY 108-3N 108-137 MHz Air Band 3 Element Yagi Antenna $159.00

Amazon.com

Brand	Sirio Antenna
Impedance	50 Ohm
Item Dimensions LxWxH	58 x 4 x 4 inches

About this item

• Type: 3 elements yagi antenna
• Frequency range: 108-137 MHz No tuning required, 108-118 Mhz @ SWR less than 1.5 ; 118-137 Mhz @ SWR less than 2.0
• Max. Power (CW) @ 30°C: 200 Watts
• Gain: 4.85 dBd - 7 dBi
• Connector: N-female with rubber protection cap

 

[vagrant]

I have found the Scantenna to be directional when I mounted it to a mast, so aim it toward the airfield. The directivity the Scantenna offers provides gain over a discone...only in that direction. Also, the elevation of either location is enough to cover the RF line of sight for the distance noted and combined definitely helps with RX. Perhaps add an FM broadcast filter inline as there may be one, several, or many, radio stations causing you RFI grief. If the FM trap filter fixes things to some degree, you can always add a 1~10 dB RX amplifier after the filter. Do not get a cheap FM filter as it can attenuate up into the frequencies you wish to monitor. Even an old Radio Shack (Archer) FM trap you may have laying around would be good enough to test initially.

Last and not least is the coax. Could it have been compromised by water/pests/wear?

 

[n6hgg]

Build a three or four element Yagi. It will actually work quite well fed directly to the coax. I have a number of Aviation four element yagis on top of my house and they are amazing. And I certainly wouldn't worry about what signals they reject. It ain't much of a problem. These things are really easy to make. Don't get wrapped up in all the rocket science. Pay attention to the correct dimensions when you make the thing and it'll work fine untested for receive duty. I have some using 50 ohm coax and others using 75 ohm TV coax. They all work great

 

[scanmanmi]

You can whip up a tape measure antenna before committing. I made one and it actually worked. You'll have to calculate the air band length.

The Tape Measure Antenna

The Tape Measure Antenna: Are you looking for an easy kid friendly project? Are you looking for a project to encourage a new generation of ham radio operators? This project is unusual, so it’s something that will certainly capture the attention of anyone, particularly a kid…

www.instructables.com

 

[PACNWDude]

About 25 miles, I manage to get that with an old Diamond discone mounted to my roof and line-of-site (close to anyway). Will have to check all the tines to make sure they are all there after the current winter storm passes though. Great antenna that has worked well for about thirteen years so far. I have several airstrips/airports within 25 miles of my location, using both Technisonic TiL-91-DE transceiver and Realistic Pro-2005/6 scanners as receivers.

 

[mlfd612]

Sirio WY 108-3N 108-137 MHz Air Band 3 Element Yagi Antenna $159.00

Amazon.com

Brand	Sirio Antenna
Impedance	50 Ohm
Item Dimensions LxWxH	58 x 4 x 4 inches

About this item

• Type: 3 elements yagi antenna
• Frequency range: 108-137 MHz No tuning required, 108-118 Mhz @ SWR less than 1.5 ; 118-137 Mhz @ SWR less than 2.0
• Max. Power (CW) @ 30°C: 200 Watts
• Gain: 4.85 dBd - 7 dBi
• Connector: N-female with rubber protection cap

View attachment 132464

I love this, but it won't be in my budget for a while.
Do you know if there are any lower priced Yagi's available that could be modded for the air band (tuned to around 127 MHz?
I was thinking maybe a 2 meter or FM TV Yagi with a filter may work, and many are lower priced.
I'm not a HAM operator and not well versed in antenna theory past the basic scanner/receive stuff.

 

[iMONITOR]

If you have strong FM broadcast radio stations in your area you might want to consider a FM-NOTCH Filter. Those signals can cause a lot of interference to the VHF AM air band even though they are FM. This results in desensitizing your receiver.

Specifications

Frequency: 88 MHz to 108 MHz
Type: 5 Pole Chebyshev
Impedance: 50 Ohms
Notch: > 45 dB @ 98 MHz
Insertion Loss: < 1 dB @ 500 MHz
Connectors: BNC (M/F)
Optional connectors: N-type (F/F), TNC, SMA (F/F)
Size: 2 x 2 x 1-1/4 Inches
Die cast aluminum, Black powder coating

Receive Filters

There are cheaper ones available on Ebay and Amazon, as cheap as $16.00 but I don't know how effective they are.

 

[iMONITOR]

Here is another excellent source for FM-Notch Filters:

PAR Electronics

PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

PAR Electronics Inc. Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave

www.parelectronics.com

Scanner Filters FAQ

Scanner Filter FAQ | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

PAR Electronics Inc. Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave

www.parelectronics.com

 

[prcguy]

I'm about 16mi line of sight to a major airport but the path is blocked by the side of an 1,100ft hill. I can pick up all airport comms and aircraft on the ground using a Discone about 25ft off the ground.

 

[iMONITOR]

I'm thinking some kind of directional antenna? Question is two fold then, I suppose:
1. Discone etc. vs. directional or amplified antenna, and...
2. Would it really be that much of an improvement, or is it an unreachable pipe dream at that distance?

Thanks all!

Just for kicks try the supplied back of set telescoping antenna extended to approximately 20-22" and see how that works. This would eliminate coax loss, overloading, and other factors that may have a negative effect on reception performance. You might be surprised!

 

[GROL]

Here is another excellent source for FM-Notch Filters:

PAR Electronics

PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

PAR Electronics Inc. Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave

www.parelectronics.com

Scanner Filters FAQ

Scanner Filter FAQ | PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

PAR Electronics Inc. Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave

www.parelectronics.com

View attachment 134684 View attachment 134685

In that response curve, 10 db attenuation at about 110 Mhz is quite significant. That is about a 90% reduction in signal. Trying to interpret that curve it looks like 118 MHz may be 3 db down. Unless someone is within a mile or less of a strong FM broadcast station there shouldn't be a problem unless the receiver is crap.

If you deploy a directional antenna, keep in mind you can wipe out gain with feed line losses. Do not use an RF amplifier at the scanner. Only use a low noise antenna amplifier at the antenna to overcome feed line losses if needed, and no more antenna amplifier gain than necessary to overcome feed line loss. The scanner does not need help from an RF amplifier at the antenna jack. Modern scanner radios are very sensitive and cannot benefit from an RF amplifier. That will only make the signal to noise ratio worse.

Keep feed line as short a possible and use low loss cable. This online calculator will give you coax feed line loss for a given type and length.

Coaxial Cable Attenuation and Power Handling Calculator | Times Microwave Systems

Calculate the attenuation and power handling capability of coaxial cables using the online calculator by Times Microwave Systems. Choose from a variety of cables and get started now!

timesmicrowave.com

Every 3 db of loss or attenuation cuts the signal in half.

 

[majoco]

I made a 300ohm folded dipole cut for 125MHz from a cut-down TV antenna - it's mounted about 15 feet off the ground and it easily receives Ohakea Tower about 40km/24miles away and I'm in a valley. The matching balun is in the orange box. The other dipole is cut for 75MHz with a balun in the pill box which is quite active around here and the discone covers everything else!

 

[mlfd612]

Ok guys, just being real here...I'm not going to be building any tape measure yagis or any other antennas. I'm just not that type of Elmer dude, lol.
That said, the scantenna did a little better when aimed in the azimuth of the antenna farm at the airfield, but I'm still not picking up Tower and Ground transmissions from the field.

I think the Sirio yagi is best, but not in my budget right now.

Would this work (using an FM broadcast block filter)?
It says it covers VHF 40-230.

Thanks!

Amazon.com