Toroid questions

[jaymot]

I live less than 10km away from an airport with no hills in between but I'm not able to receive any air traffic using a simple telescopic whip. I've decided to build the VHF airband dipole from VK1NAM (instructions here) and hang it in an open upstairs window facing the direction of the airport. I may still not be able to hear the ground crews or tower but I might be able to start picking up something. I can get all the parts except for the T50-17 12.8mm toroid core. I can get cores such as T50-6, T37-2, T94-2, T25-6 and T68-2, just not that particular one called for in the parts list. My questions are: does it matter what core I use as long as it's iron or contains iron powder? Could I just use a 300 ohm to 75 ohm TV balun instead? That would make for a lot easier construction.

FYI this site shows the specs of the various toroid cores.

Thanks.

Edit: I can't find any contact information for the author of the antenna plans and don't know where else to ask my questions.

 

[merlin]

The TV baluns are what I use for 1/2 wave RX dipoles, they are not ideal but fine for receiving scanning etc.

 

[prcguy]

The air band dipole project is a half wave and about 70 ohms in free space or less when near the ground or other stuff. The balun in the project appears to be a 1:1 choke type and you don't want to use a TV balun because those are a 4:1 type.

A choke balun is not mandatory but if you still want to use one three snap on ferrite beads with a 43 mix over the coax will provide good common mode rejection.

 

[jaymot]

So I'd just solder the center conductor of my coax jack to one element, the shield to the other via a solder tab as shown, then add a few ferrite clamp-on chokes to the coax near the antenna?

I wonder how much the impedance drops when near other objects as mine would be hanging indoors in front of an open window but not that far away from concrete walls? Should I plan on using RG6 or RG58 coax? I want to get as much "oomph" from the antenna as possible given the conditions here.

 

[prcguy]

Yes, exactly. Do you have an antenna analyzer to check the VSWR?

So I'd just solder the center conductor of my coax jack to one element, the shield to the other, then add a few ferrite clamp-on chokes to the coax near the antenna?

 

[jaymot]

No but I could get one. It would greatly increase the cost of the project, but I suppose now that I'm (having fun) constructing antennas it would be a good investment.

Edit: It looks like the only one I can get here that covers the airband is the Mini1300, the rest are HF only or 2 meters and up. (I take that back, there's also the NanoVNA and it's cheaper.)

 

[jaymot]

Just a follow-up: I finally got around to building the airband dipole and hanging it in an upstairs window that faces east toward the airport. Also I got a NanoVNA v2 Saturday before last. I haven't gotten around to checking the VSWR or adding ferrite chokes yet. It's connected to 10 meters of RG6 coax. Compared to the mag mount and single telescopic whip that came with the NESDR SMArt the ATIS information is understandable vs. barely audible. The tower's still kind of weak but more than before understandable. (I've heard an Air Hongkong pilot complaining of the same thing yesterday afternoon, that the tower was "quiet", so I think the problem's at the control tower's end. The planes on the ground come in clearer and stronger.) I'm not even using an FM broadcast bandstop filter. I get less interference from them with a more resonant antenna. Still, I plan to add a Nooelec Flamingo + FM filter to the circuit when it arrives.

 

[jaymot]

Follow-up to my follow-up: I moved the airband dipole to another location, on the second-floor terrace in the SE corner of my house, and it's working a lot better there. The VSWR's rather bad at around 2:1 but ATIS, the tower and everything on the local frequencies is booming in where all but the coms coming from airplanes aloft were fairly weak and unreadable at the old location. I'm even hearing planes taxiing to the runway loud and clear. I extended both telescopic whips to around 55cm and am still feeding it with good-quality RG6 though I may replace it with LMR400 to see if that improves the VSWR or makes it worse, or I may just leave it alone. "If it ain't broke don't fix it."

 

[Ubbe]

The VSWR's rather bad at around 2:1

Those VNA's are usually designed to operate at 50 ohm, and using 10 meters of 75 ohm RG6 will get some different results if you do not use an impedance matching pad. When testing out the antenna you could use a short run of RG58, a meter or two, and make the VNA loop a live measurement while handling the coax in different angles to see what it does to the result and get the best measurement.

The element that goes to the braid of the coax then goes to ground at the receiver. I would think that actually using the signal from that element and not only have it as a counterpoise would get a better signal. A 1:1 balun transformer could be a solution, or a 1:2 having the 100 ohm end to the antenna will make it more wide banded and suitable to cover more of the large VHF airband. Even those 75/300 1:4 baluns will increase bandwidth even more but to the expense of getting lower signals at the antennas actual tuned frequency.

/Ubbe

 

[jaymot]

@Ubbe I think that as long as I can receive air traffic including the tower with enough signal strength and quality that I can understand what's being said I'm going to leave it well enough alone for now. On the other hand I do have a 3 meter length of RG58 that I could use. Nothing shorter than that though. I'll keep your advice in mind though. Knowing me as I do I'll probably want to see if I can tweak the antenna or coax to make it more efficient, sooner or later.

Thanks.