UHF VHF tv antenna design

[robsims]

Hi,

I'm a beginner in electronics. I want to design a VHF/UHF tv antenna in the frequency range from 40 to 800 mhz. What are the most reliable webpages to start with?
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[selcuk]

This one is very useful to start with:

https://www.antenna-theory.com/

[Benta]

Normally you'd split that on two antennas or even three:
VHF + UHF or VHF1 + VHF2 + UHF, all of them Yagi types.

[robsims]

I have a cheap chinese yagi combination VHF/UHF antenna which operates at 45 to 860 mhz. Input impedance is 75 ohm and a gain of 26 db for VHF and a gain of 28 dB for UHF. The antenna is good, but the boom and elements are from weak cheap thin aluminum tubes. The antenna also has a preamp. I want to design/build this antenna with sturdier thick aluminum or copper tubes. I need a  reliable calculator to build it or if someone has antenna design information i can calculate it on my own

[radiolistener]

I want to design a VHF/UHF tv antenna in the frequency range from 40 to 800 mhz. What are the most reliable webpages to start with?

The first thing that you're needs to start with is that antenna VSWR, Q and BW parameters are linked together.

BW = (VSWR-1) / (Q*sqrt(VSWR))

If you improve one of them, you will get worse other parameters and vice versa.

And since you're needs to keep max VSWR <= 2.618 in order to keep 3 dB attenuation within specified bandwith, the only option to increase bandwidth is to reduce Q factor of antenna. It means that the only option is to reduce efficiency of your antenna.

40 to 800 MHz is 760 MHz bandwidth, this is very wide. And it means that such antenna will be incredibly inefficient.

If you want to get efficient antenna, you're needs to reduce bandwidth as narrow as you can.

For TV antenna, the best efficiency you can achieve for bandwidth equals to one TV channel, because you cannot use more narrow bandwidth.

[robsims]

No i want a combination antenna with VHF and UHF on it. Just like the chinese antenaa i described in one of my replies

[wasedadoc]

I have a cheap chinese yagi combination VHF/UHF antenna which operates at 45 to 860 mhz. Input impedance is 75 ohm and a gain of 26 db for VHF and a gain of 28 dB for UHF.

If you believe those numbers you will believe anything.

But if it working well enough for you just copy the length and spacing of all elements and construct from slightly thicker metal.

[radiolistener]

Just like the chinese antenaa i described in one of my replies

Most of Chinese antennas are just a piece of crap. They can sell you rusty nail in a plastic tube and write that this is wideband antenna from 1 MHz to 1000 MHz. But that is not antenna but rusty nail 

Good antenna is narrowband antenna. If you see that antenna is wideband, then this is not antenna but a rusty nail...

Good antenna can have several resonant frequencies (for example one on VHF and one on UHF band), but it should have narrow bandwidth around each resonant frequency.

[robsims]

THanks, but if i copy the length and spacing of the elements exactly and i use thicker,larger diameter tubes, will the thickness and larger diameter of the tubes (aluminum or copper)  not affect the antenna in a bad way?

[robsims]

Ok thanks, but in you're opinion  how much antennas do i need to cover 45 to 860 mhz?

[radiolistener]

THanks, but if i copy the length and spacing of the elements exactly and i use thicker,larger diameter tubes, will the thickness and larger diameter of the tubes (aluminum or copper)  not affect the antenna in a bad way?

Change in element diameter will change antenna properties, it can change frequency and bandwidth.
But if change is very small (for example less than 1/1000 of wavelength), it should not affect it much.

But at 800 MHz it will be pretty sensitive to even small change, because wavelength is just 0.37474 meters.


Using copper tubes will increase antenna Q and antenna will be a little bit more efficient than the same antenna from aluminum tubes. But at the same time copper tubes will reduce antenna bandwidth. The difference will not be critical, because difference between aluminum and copper resistance is not so high.

Ok thanks, but in you're opinion  how much antennas do i need to cover 45 to 860 mhz?

It depends on how many frequencies you're want to receive

Also note that letter register for units is important. Hertz are always written with capital letter.
And mHz and MHz are different units.

mHz means milli-Hertz (0.001 Hz), while MHz means Mega-Hertz (1000000 Hz)

[Bud]

Good antenna is narrowband antenna. If you see that antenna is wideband, then this is not antenna but a rusty nail...

It depends on the use case. For ultrawide band radar, aperture radar or georadar applications it is required to have a wideband antenna.

[radiolistener]

It depends on the use case. For ultrawide band radar, aperture radar or georadar applications it is required to have a wideband antenna.

yes, because such radar needs wide bandwidth.
As I said, for best efficiency antenna should have as narrow bandwidth as possible for your use case.

For TV it will be bandwidth of one TV channel.

[CaptDon]

Get yourself a copy of the A.R.R.L. 'Antenna Handbook' , learn some theory and then re-design / modify the piece of crap that is falling apart. The diameter of the tubes effects bandwidth of each tube. Yagi's have a driven element (the receive element) a reflector and a bunch of directors. Basically the longer the boom and the more director elements the higher the gain and the higher the directivity. Read and learn.

[robsims]

You've got it right CaptDon. That is exactly what i want to do . I want to learn some theory and then redesign that crappy chinese antenna with sturdy material. I already downloaded the ARRL, antenna handbook 19th edeition.  I saw some online website calculators for antenna's. If someone who is reading this know which website with a calculator is reliable please post the link

[radiolistener]

if you want calculator, you can use MMANA-GAL or other NEC based simulator, it allows to calculate antenna parameters from provided wire geometry.

[robsims]

I'm going to design two antenna's. A VHF 40 - 200 MHz antenna and a UHF 400 - 700 antenna. Is it possible to combine these 2 antennas on one boom?

[calzap]

Just like the chinese antenaa i described in one of my replies

Most of Chinese antennas are just a piece of crap. They can sell you rusty nail in a plastic tube and write that this is wideband antenna from 1 MHz to 1000 MHz. But that is not antenna but rusty nail 

Yeah.  I’ve seen Chinese yagis where the driven elements were not insulated from the boom.  In other words, the antenna was a short at the end of the cable.  I’ve seen others where the center line of the coax connected to a single full-width element that was insulated from the boom.  And the shield conductor connected to the boom and other elements.   These antennas are sold on Amazon and eBay for WiFi and remote A/V use.

Mike

[robsims]

see picture, this is the antenna i want to build. It also uses a preamp. The antenna has a boomlength of 60 cm and is 80 cm wide. The materials they made the antenna of a are very cheap and weak. I want to try do make adjustments and use better, sturdier materials. I've looked at a lot of websites with information and antenna calculators, like this one  https://www.steeman.org/Antenna/Yagi-Antenna-Calculator.     If anyone knows a reliable source with info about how i can build this antenna please post it, many thanks in advance

[MarkT]

I've used NEC2 in the past: https://www.nec2.org/

[Benta]

see picture, this is the antenna i want to build. It also uses a preamp. The antenna has a boomlength of 60 cm and is 80 cm wide. The materials they made the antenna of a are very cheap and weak. I want to try do make adjustments and use better, sturdier materials. I've looked at a lot of websites with information and antenna calculators, like this one  https://www.steeman.org/Antenna/Yagi-Antenna-Calculator.     If anyone knows a reliable source with info about how i can build this antenna please post it, many thanks in advance

Mechanically, that's pretty easy to make, I've built Yagis myself.

Use a square 20 x 20 mm aluminium tube for the boom, and 10 mm rund aluminium tube for the directors. Drill 10 mm holes in the square tube, stick the directors through, and secure with self-cutting screws (drill small cross-holes before).

Make the reflector in the same way.

But I have an issue with the big "X"-thing, which seems to be the VHF part. Take that away (make a separate VHF antenna).
And I also have an issue with the active element (the oval-formed alu-tube), which is obviously 300 ohms. You should redesign that to a simple dipole, which will be 75 ohms.

Concerning the preamplifier, well, "maybe yes, maybe no". It will compensate for cable loss between antenna and receiver, but also introduce noise by itself. It needs experimentation.

Also, check if you really need channels 2...4 (low VHF). Limiting your VHF antenna to the high VHF band (around 200 MHz) makes things much easier.

Last, use the Yagi calculators others have linked to in earlier posts. You may have to reposition the elements when removing the VHF "X".

[Terry Bites]

Log periodic antennas are great for wideband VHF/UHF.
Check out this page: hamwaves.com/lpda/en/index.html

In the UK they are very popular for home DVB-T and DAB reception which use both the VHF and UHF bands.
A Yagi is a special case of a log periodic array. Not very boadband.

[wasedadoc]

A Yagi is a special case of a log periodic array.

Not really true.  A Yagi has only one driven element whereas all elements of a log periodic are driven.