TV aerial problems

[Onoff]

I’ve ordered the Practical Antenna ( ) Handbook by Carr. Apparently a good bridge between A-level physics and the ARRL book.

I built my corner reflector aerials for Band A TV reception from info gleaned in that book & others:

 

[ajlelectronics]

I built my corner reflector aerials for Band A TV reception from info gleaned in that book & others:

View attachment 480220

I have been trying to find my VHF/UHF manual. I had two and gave one away, but now can't find the other! ('indoors tidying up, I found the Radcom handbooks in the laundry basket).

 

[voipio]

View attachment 480216
All the ones I’d seen before had the middle one above. Can anyone tell me how that one works?

RF can be "funny stuff" to work with. The printed circuit track between the two ends of the antenna could basically be a delay line, where the 180 degree out of phase signals at the two antenna terminals end up being in phase at the right hand antenna teminal in the middle image. The "ground" copper surrounds the "wiggly" track, and may help to perform some sort of impedance transformation. I suspect there were a good few prototype variations.

@voipio you mentioned transmitter groups. Rosemarkie is B, but aerials seem to come in A or K.

A Group B antenna would seem to be the one best suited to Rosemarkie's transmissions. Group K would be a 2nd choice.

 

[Onoff]

A Group B antenna would seem to be the one best suited to Rosemarkie's transmissions. Group K would be a 2nd choice.

Plenty of A,B & K options here:

Mine's a whopper. The XB16A. I'm in a valley, extreme fringe area and have it pointed at Crystal Palace. On a pole up hill about 30m from the house.

 

[Onoff]

Just playing here...

618 - 674MHz is the Rosemarkie band unless I'm wrong. So 646MHz dead centre.

I plumbed that into an online corner reflector calculator. (I have a headache and couldn't be bothered to do the maths, will find my scribblings later from when I made mine). Page 8.23 & 8.24 in the VHF UHF Manual explain. P404,405,406 in the Practical Antennae Handbook.

https://3g-aerial.biz/en/online-calculations/antenna-calculations/corner-reflector-antenna-online-calculator

Gives you all the measurements to make one. The reflector can be made from galvanised mesh rather than sheet. I've done one in ali sheet and another in 1/2" galv mesh.

I'll try and get some pics of the neighbours ones I made later. Both work a treat.

 

[Guineafowl]

RF can be "funny stuff" to work with. The printed circuit track between the two ends of the antenna could basically be a delay line, where the 180 degree out of phase signals at the two antenna terminals end up being in phase at the right hand antenna teminal in the middle image. The "ground" copper surrounds the "wiggly" track, and may help to perform some sort of impedance transformation. I suspect there were a good few prototype variations.

Thanks. I’ll see what I can find out on Physics Forums as well, but they can be touchy sods.

From what I can find out, a folded dipole in a Yagi setup has an impedance between 10 and 40 ohm, which suggests a step-up impedance transformation.

Just playing here...

618 - 674MHz is the Rosemarkie band unless I'm wrong. So 646MHz dead centre.

I plumbed that into an online corner reflector calculator. (I have a headache and couldn't be bothered to do the maths, will find my scribblings later from when I made mine). Page 8.23 & 8.24 in the VHF UHF Manual explain. P404,405,406 in the Practical Antennae Handbook.

https://3g-aerial.biz/en/online-calculations/antenna-calculations/corner-reflector-antenna-online-calculator

Gives you all the measurements to make one. The reflector can be made from galvanised mesh rather than sheet. I've done one in ali sheet and another in 1/2" galv mesh.

I'll try and get some pics of the neighbours ones I made later. Both work a treat.

I did ring aerialsandtv.com, but they won’t post up here.

My Practical Antenna Handbook is 6th ed. 2025, and only has a short bit on corner reflectors in the microwave section, p.446. No dimensions or calculations, just a sketch.

 

[ajlelectronics]

From what I can find out, a folded dipole in a Yagi setup has an impedance between 10 and 40 ohm, which suggests a step-up impedance transformation.

No, a resonant dipole is around 75 Ohms. If you fold it back on itself, the impedance goes up to around 300 Ohms. Adding reflector and directors drops it again to around the 75 Ohm mark, the elements are spaced to achieve the target impedance.

A BA lun is a Balanced to UN balanced transformer. It doesn't do anything with impedance matching, it is just there to enable the balanced (about earth) dipole to match the unbalanced feeder and tuner input.

The printed circuit in the connector box that you see is that animal. What you have to remember is that DC resistance and RF impedance are two different things. Take a capacitor in parallel with an inductor; at DC it always looks like a near dead short, low impedance off resonance, but at resonance it will look like an extremely high impedance. A series arrangement is the opposite at resonance. These characteristics are made use of in the PCB balun arrangements.


I have edited that a number of times, I just can't get my brain to function today, so I may have to revisit the post.

 

[Guineafowl]

No, a resonant dipole is around 75 Ohms. If you fold it back on itself, the impedance goes up to around 300 Ohms. Adding reflector and directors drops it again to around the 75 Ohm mark, the elements are spaced to achieve the target impedance.

I can see why RF is known as the dark arts of electronics. Try as I might, I can’t find that information in either book. It must be available somewhere, for those designing their own DTV Yagis. Either that, or done by trial and error with a vector network analyser. I have a nanoVNA, but again, can’t find good info on hooking it up to a Yagi to tune.

I’m ok with impedance and series/parallel LC circuits. We did some of that at A-level, and there’s a section on it in the very good Practical Electronics for Inventors. Going from that to aerial/RF stuff is a bit of a jump, though!

 

[Guineafowl]

While I’m on the case, where can I drill this double-glazed window for the new coax? Existing piece is very hard to pull through, and I doubt I can use it as a pull cord.

 

[ajlelectronics]

While I’m on the case, where can I drill this double-glazed window for the new coax? Existing piece is very hard to pull through, and I doubt I can use it as a pull cord.

View attachment 480468

You don't ideally. Go to the brick wall and drill that inside to out at an angle (down) As you get near the outside, turn off the hammer to minimise blowout. You can use a plastic "blowout" plate to cover the entry point. There are some who advocate drilling from outside to in, but that means drilling uphill and the chance of a large area of plaster coming away; much harder to repair.

 

[Guineafowl]

You don't ideally. Go to the brick wall and drill that inside to out at an angle (down) As you get near the outside, turn off the hammer to minimise blowout. You can use a plastic "blowout" plate to cover the entry point. There are some who advocate drilling from outside to in, but that means drilling uphill and the chance of a large area of plaster coming away; much harder to repair.

Unfortunately, it’s an old stone croft house, and the wall is 700mm thick. I’d rather not drill it if I don’t have to.

 

[ajlelectronics]

I can see why RF is known as the dark arts of electronics. Try as I might, I can’t find that information in either book.

Search for "Folded dipole" and there are lots of hits, from nonsense to deep theory. Two things to bear in mind though, we are talking about impedance AT RESONANCE and also in free space. Real world factors will influence those results.

Here is just one example, with no endorsement as I have not read it properly.

VHF/UHF Yagi Antenna Feed Design - K7MEM

VHF/UHF Yagi Antenna Feed Design

www.k7mem.com

 

[ajlelectronics]

Unfortunately, it’s an old stone croft house, and the wall is 700mm thick. I’d rather not drill it if I don’t have to.

We would be at odds then. I would walk away from a job where I was required to drill any window or door frame. It will usually come back to bite you, shattered glazing units and damp ingress are just two concerns.

 

[Guineafowl]

We would be at odds then. I would walk away from a job where I was required to drill any window or door frame. It will usually come back to bite you, shattered glazing units and damp ingress are just two concerns.

Ok. I guess I could try using the old coax as a pull-through.

 

[Blobber]

Ok. I guess I could try using the old coax as a pull-through.

Pull the old one out and make the existing hole a bit bigger

 

[Mark 5]

We would be at odds then. I would walk away from a job where I was required to drill any window or door frame. It will usually come back to bite you, shattered glazing units and damp ingress are just two concerns.

While I understand your reasoning in most cases you would be being over cautious.

 

[Mark 5]

Ok. I guess I could try using the old coax as a pull-through.

Either remove the DG glass unit by removing the beading and drill the frame with a sharp spade bit, you could do this without removal, no hammer action.

Or easier still if you can't use existing as a pull through, cut coax flush both sides of hole and with same said trustee spade or a masonry bit just drill through to clear remnants of old coax.

Make sure you create a drip loop in the coax outside, and use good coax.

 

[Onoff]

 

[Onoff]

My Practical Antenna Handbook is 6th ed. 2025, and only has a short bit on corner reflectors in the microwave section, p.446. No dimensions or calculations, just a sketch.

I'll pm you.

 

[Guineafowl]

Pull the old one out and make the existing hole a bit bigger

No, I think the coax was built in when the window was installed. It doesn’t follow a straight path.