Folded dipole for aviation monitoring

[w1av]

Hello

I want to construct a FOLDED DIPOLE for monitoring the AVIATION BAND. I want to have the antenna resonant at 130 MHZ. I am unsure of the DISTANCE between the folded elements. 7 feet is the exact length for a 1/2 wave antenna on 130MHZ. I need help with the folding dimensions. I looked online and find nothing helpful. I am tempted to email the manager at local airport to see if they have any small monitoring antennas in their junk pile. It would be easier to just make a 1/2 wave dipole but I need smaller antenna. Folded dipole would be perfect. I made a cheap and quick 1/2 wave vertical dipole with 2 three foot elements mounted on a small piece of wood and fed with 75 ohm coax. It is mounted on side of house just below roofline. A temporary setup. Reception is GREAT. I live approx. 5 miles as the crow flies northwest of the state airport which gets very busy (in Rhode Island). My radio is an old Uniden bearcat BC8500xlt. I use this temporary antenna for ACARS as well and it receives well. The antenna is pretty ugly but works extremely well.
Anyway if someone could help me with the dimensions for a folded dipole for 130 mhz it would be great! Bob

 

[prcguy]

A half wavelength at 130MHz is more like 3.6ft. The spacing is probably not critical and something between 2 and 4 inches would probably work. A folded dipole has a feedpoint impedance around 200 ohms so you will need something to match it like a TV transformer.
prcguy

Hello

I want to construct a FOLDED DIPOLE for monitoring the AVIATION BAND. I want to have the antenna resonant at 130 MHZ. I am unsure of the DISTANCE between the folded elements. 7 feet is the exact length for a 1/2 wave antenna on 130MHZ. I need help with the folding dimensions. I looked online and find nothing helpful. I am tempted to email the manager at local airport to see if they have any small monitoring antennas in their junk pile. It would be easier to just make a 1/2 wave dipole but I need smaller antenna. Folded dipole would be perfect. I made a cheap and quick 1/2 wave vertical dipole with 2 three foot elements mounted on a small piece of wood and fed with 75 ohm coax. It is mounted on side of house just below roofline. A temporary setup. Reception is GREAT. I live approx. 5 miles as the crow flies northwest of the state airport which gets very busy (in Rhode Island). My radio is an old Uniden bearcat BC8500xlt. I use this temporary antenna for ACARS as well and it receives well. The antenna is pretty ugly but works extremely well.
Anyway if someone could help me with the dimensions for a folded dipole for 130 mhz it would be great! Bob

 

[ko6jw_2]

I suggest that you get an ARRL Antenna Book and read the section on folded dipoles. The spacing of the elements will affect the feed point impedance and the bandwidth. Any dipole presents a balance load and should be fed with a balun or a tuner.

The vertical dipole should be mounted in such a way as to keep the antenna a quarter wavelength from the support and the feed should also run at right angles from the antenna for a quarter wavelength.

Using 450 ohm ladder line cut to approximately one half wavelength taking into account the velocity factor and shorted at both ends should work. Attach the feed in the center on only one wire.

You are receiving only - so the dimensions are not critical as long as they are in the ball park.

 

[majoco]

I made mine from a cut down TV antenna - there's plenty of those around after we went to only UHF. I kept the balun and the terminal box, just shortened the elements. I did make a basic mistake - shouldn't have used the metal clamp at the junction as it makes it into a 37.5 ohm antenna, so now it's insulated. Works well.

There's something wrong with your sums - each leg is a quarter wavelength, not half. The bits I used are just the loops, there are two, only one shown in the top pic.

 

[prcguy]

Looking at the bottom picture in majoco's post, if you ground the unterminated end of one element to the boom then attach coax center to the other unterminated end, coax shield to boom and leave the grounding boom clamp in place, it will retain the broad band characteristics of a folded loop and match ok to 50 ohm coax.
prcguy

I made mine from a cut down TV antenna - there's plenty of those around after we went to only UHF. I kept the balun and the terminal box, just shortened the elements. I did make a basic mistake - shouldn't have used the metal clamp at the junction as it makes it into a 37.5 ohm antenna, so now it's insulated. Works well.

There's something wrong with your sums - each leg is a quarter wavelength, not half. The bits I used are just the loops, there are two, only one shown in the top pic.

 

[majoco]

if you ground the unterminated end of one element to the boom.......and....... leave the grounding boom clamp in place

Doesn't that mean that one leg is connected to the boom at both ends....?

My original idea had only one connection to the boom at the metal clamp - the two other ends of the legs went to the primary of the 300:75 ohm transformer - I thought this was quite logical.

 

[prcguy]

Yes, one side will be a loop that goes from the boom outward and returns to the grounded boom. The other side will have one end grounded to the boom and the other floating to attach the center of the coax.

Most stacked loop dipole arrays are made like this.
prcguy

Doesn't that mean that one leg is connected to the boom at both ends....?

My original idea had only one connection to the boom at the metal clamp - the two other ends of the legs went to the primary of the 300:75 ohm transformer - I thought this was quite logical.