Analog UHF TV signal reception improvement idea

[audioguru]

CITY tv ......

Remember when CITY TV started and used channel 79?
I liked their Friday Baby-Blue movies.

When was CITY TV sold to Rogers Cable TV Company?
What happened to rock and roll oldies station 1050 CHUM AM? Now they are the news audio from CITY TV but it sounds very muffled because they don't use equalization for an AM radio station. They are owned by CTV?
Bell owns CTV?
The teachers union wants to buy Bell?

 

[blueroomelectronics]

I listen to internet radio like shoutcast or itunes. I guess AM DAB died awhile ago.

 

[HSter]

I built it!

1. Essentially you make two triangles...

Hi Tesla!

I've built the pyramidal horn with two sides as you described. I used a coax balun with 4:1 ratio, but the signal reception though much much better than before is a bit weak (dots on the video, audio very clear).

So, first of all Hats off to You, Thanks!

Secondly, to improve on this, must the antenna be at a certain height above ground?

Thanks once more.

 

[HSter]

I can give you basic measurements for singel element aerial with a photo that I use and I also have a Kingray masthead amplifyer 22Volts ac and a digital set top box which gives me a very good reception.
Lt me know If you whant this Informtion.
The other way is If there is a hill close to you, you can use a passive aerial method.
I hope I can help you out
Greg

Thanks, that's nice of you.

Yes, i would like to know more.

Does the ant. and amp have to be used together?
Does the amp work only for a given range of frequencies, the one channel I'm keen on getting is UHF channel 32 (f=581MHz)

 

[HSter]

I'm just that close to buying a Yagi for the attic.

Why not build one yourself?

 

[HSter]

you can make a yagi out of loops of wire...

Thanks for your response unclejed613.

What materials can be used as the element wires?

 

[unclejed613]

you can feed that amp with DC to reduce the noise. the voltage isn't too critical' some older amplifiers only had half wave rectifiers, so with 22Vac, you got 12-15VDC for powering the amp. those amps would get really squirrely when the electrolytics dried out, and the amp, if it didn't oscillate, would give you hum bars in the picture because the amplifier was operating off of unfiltered half wave rectified power. in the base unit for those amps is a filter that allows the low frequency AC to go up to the amplifier, and splitting off the RF that goes to the TV. the input of the amp isn't very critical as far as what you feed it with. you will get some gain loss with impedance mismatches, but you will still get amplification. most center fed antennas average 30 to 100 ohms impedance, and mismatches to the 75 ohm input of the amp in that range aren't serious. when you start feeding with asymmetrical antennas, then you start to get serious mismatches. one thing that will help to "wideband" the antenna is a large diameter driven element. the impedance of the antenna remains the same, but the "radiation resistance" goes down, which means more signal gets coupled to the antenna. that's why discone antennas work so well, the diameter of the driven element is very large.

antenna height with relation to radiation resistance and altering the impedance match only is a factor when the antenna is less than a wavelength from the ground, and at 500Mhz or so, that's a distance of just a bit more than half a meter.

 

[blueroomelectronics]

Why not build one yourself?

It's cheap to buy the Chinese copies. The DB4 bowtie is only $20, I'll check the price of the Yagi. The hard part is mounting it and height is a big help.

 

[Tesla23]

Hi Tesla!

I've built the pyramidal horn with two sides as you described. I used a coax balun with 4:1 ratio, but the signal reception though much much better than before is a bit weak (dots on the video, audio very clear).

So, first of all Hats off to You, Thanks!

Secondly, to improve on this, must the antenna be at a certain height above ground?

Thanks once more.

That's great - it does give great results for little effort.

Now for some of the other details of radio reception (you are progressively opening a can of worms..)

How to improve your signal further:
1. Height - very important. If the land around you is flattish then each time you double the height of the antenna you get 6dB more signal. (If you are on top of a hill then the local height matters less.) So long term there may be a case for making a ruggedised external antenna and mounting it at the top of a pole. Be careful that you don't lose the extra signal you get in cable losses. If your cable loss is significant (look up cable specs) then you can use a masthead amplifier.

2. Preamplifier - most TV sets don't have a good noise figure. You can generally get better performance by using a low noise amplifier at the output of the antenna. Be careful though if there are any strong signals around within the bandwidth of the antenna / amplifier as you may overload the amplifier or TV and degrade performance (filtering may be the answer in these cases).

3. local obstructions - try to make sure that your antenna has as unobstructed view toward the transmitter as possible, if indoors try to point out of a window instead of through a brick wall, close by trees etc.

4. Polarisation - just checking that you understand the polarisation of the horn and your signal, if not, try rotating the horn through 90 deg (still pointing at the tx) and see what happens.

5. Gain - if it happens that you can fit in a bigger horn then go for it.

At this stage, once you have found the best place for your horn, playing with a preamp may be the best bang for buck. I have used a MiniCircuits monolithic amplifier (something like an ERA-3) at times, these are pretty easy for you to play with, you can experiment with the RFC (wind your own is OK) and all you need are two blocking capacitors. I don't know what is available locally for you. You can make discrete transistor ones, there are circuits around, probably a bit lower noise but also more complex.

Enjoy the TV!

 

[HSter]

Thanks again Tesla23.

I would like to give the amps a shot, esp. the RFC.

I am located near a huge rock (about 100-200m high), and the transmitting station is about 50km (30miles) away.

 

[blueroomelectronics]

Is the rock between you and the station? How tall is their tower, how many watts?

 

[HSter]

Is the rock between you and the station? How tall is their tower, how many watts?

Wow, I really don't know their details. but the rock is between.

 

[blueroomelectronics]

TV stations will tell you their power & antenna height. You may need a pretty high mast.

 

[HSter]

must there have to be line of sight?

can't i get like the signal from the pyramidal horn that Tesla23 gave me, and amplify/boost it?

 

[blueroomelectronics]

Depends on the rock. Amplifying will also boost and add noise. At 50k height will help but it all depends on the transmitter power & height.

 

[HSter]

Won't the effective impedance drop when similar elements are connected in parallel to form an array. Much like resistors connected in parallel? ...

If the above is correct, using 4 elements implies that there is no need for a balun since 300/4 = 75 ohms, the characteristic impedance of the coax transmission line. Right?

Hi Tesla23, pls can you kindly clarify these assumptions for me.

Thanks

 

[Tesla23]

Hi Tesla23, pls can you kindly clarify these assumptions for me.

Thanks

Won't the effective impedance drop when similar elements are connected in parallel to form an array. Much like resistors connected in parallel?

Yes, exactly like resistors. BUT it is tricky... If you had 4 horn antennas, the impedance at each is about 400ohms. If you had all the feedpoints together (with the antennas pointing at 12, 3, 6 and 9 o'clock) then when you connect them all together you will see about 100 ohms (it actually more complex than that - see the note at the end). Unfortunately this is not what you want to do, you probably want all 4 to point at the TV trasnmitter, so the feedpoints will be some distance apart. And with high frequencies you need to understand what you are doing in order to be able to connect wires to the terminals and make sure that the impedance is the same at the other end of the wire. If you used 400 ohm twin-wire line and used equal lengths to each horn and kept the phasing correct, then when you connect them together you will see about 100ohms.

If you connect 300ohm twin-wire lead then the impedance at the far end of the lead will be somewhere between 400 ohms and 225 ohms (with varying amounts of reactance) depending on the length of the wire.


If the above is correct, using 4 elements implies that there is no need for a balun since 300/4 = 75 ohms, the characteristic impedance of the coax transmission line. Right?

You would still need a balun, but in this case a 1:1 balun. It would work somwhat without the balun, the gain is likely to be reduced and the performance of the horn will depend on the positioning of the coax feedwire.

If you want more gain I would strongly suggest that you build a bigger horn than try to make an array of horns at this stage. Doubling D is ideally equivalent to the gain achieved by having an array of 4 horns.


Note: If the impedance of one horn antenna is 400 ohms, then when you connect two together but pointing in opposite directions you will see two 400ohm resistances in parallel and see 200 ohms. What can bite you though is the mutual coupling between the antennas. Imagine you have two resonant half-wave dipoles, way apart, each provides about 70 ohms. Connecting the two together with equal lengths of 70 ohm tx line and you see 35 ohms. Now take the dipoles and place them very close together, almost touching, and connect their terminals, now you see 70 ohms, not 35. This is because of the strong mutual coupling between the dipoles, a current flowing in one causes a current to flow in the other.

 

[unclejed613]

partial answer to the paralleled antenna impedance question is here:
https://www.electro-tech-online.com/threads/why-antenna-impedances-are-what-they-are.91861/