Building a Tuneable Tv Antenna

Intro: Building a Tuneable Tv Antenna

Working on a project about measurement of DVB-T signals i needed a tunable antenna. The problem with DVB-T in Germany is that the frequencies spread over a wide frequency range. If you like to use good antennas for every channel you have to use more than one antenna. Looking at the channel spectrum and the tv channels that are being broadcasted you will notice that some tv channels are transmitted with more power than others. A compromise need to be found to receive all channels the best way possible.
To be able to experiment with the antenna and to tune it to different frequencies, I decided to build a antenna where the elements can be changed in length.
Looking around the Internet and the "Rothammel" I found that the "sleeve antenna" is ideal. It easy to construct and the elements can be build using expendable antennas from r/c remote controls.

STEP 1: Mechanical Construction

Get some extendable antennas from r/c remote controls. I had some lying around from small r/c cars I bought on eBay ( https://www.instructables.com/id/Using-RC-car-parts-as-remote-control/ ). Using a thin coax cable (RG-174), the cable could be feed through the small pipes of the antenna.
Two antennas will be used for the sleeve antenna, being a vertical polarized dipole. The antenna will be small because it is build for a frequency range 470Mhz to 740Mhz. This makes the mechanical construction easy. Just use some heat shrinkable tubing to fix them.
The antenna can be tuned by changing the length of the dipole elements. The antenna can be mounted using a vacuum cup because it doesn't wight much.

All measurement is done in the metric system using meters not inches ... I am German ;-)

Material:
- two r/c remote control antennas
- about 40cm RG-174 coax cable (don't use to much because the damping of this cable is high)
- coax connector that can be plugged into your receiver, i used a bnc blug
- vacuum cup, for easy mounting
- hexagonal brass nut, size M3 (in my case, could be other size for you)

Tools:
- pliers
- soldering iron
- solder
- sharp knife
- hacksaw
- small screw driver
- fine sanding paper
- maybe some silicone spray or oil

STEP 2: Calculation

The antenna is basically a vertical polarized dipole antenna using 1/4 wave length elements.
The frequencies used for DVB-T in Germany contain 177,5Mhz to 226,5Mhz (band III, vhf) and 474Mhz to 778Mhz (band IV and V, uhf). The r/c antennas used here are to short for vhf but they work well with uhf. So we will build a uhf only antenna here.
The wave length of a frequency can be calculated using the formula L=c/F. c is a natural constant; c = 300 m/s . Using 470Mhz the wavelength is 0,64m. 1/4 wavelength is 0,16m. The same calculation for 778Mhz leads to a 1/4 wavelength of 0,096m. The result shows that the r/c antennas fit our needs.

The above calculation is not really exact. There is a shortening factor v that depends on the dielectric and the thickness of the elements of the antenna. Rothammel contains a diagram where you can take the shortening factor for a dielectric of air if you got the thickness of the elements. In our case we have to use a mean thickness. Measure the thickness and calculate the thickness ratio. The thickness ratio is defines as follows Ld = length / diameter. In my case the mean diameter is 5mm. The thickness ratio is 127,66 for 470Mhz. Now we can take the shortness factor from the diagram. It is about 0,96 for 470Mhz. The correct length of the element can now be calculated: l = c / F * V / 4 . This leads to 15,32cm for 470Mhz and to 9,2cm for 778Mhz. For the mean frequency of the uhf band of 626Mhz the length is 11,41cm.

STEP 3: Preparing the R/c Antennas

Remove the antennas from the r/c remote controllers.

One antenna need to be shortened, because we like to feed the coax cable through it. In my case I needed to cut the two thinnest elements. To find out how much you need to cut, just look for the smallest elements that is big enough to contain the coax cable. Now the mounting base need to be cut off completely so that the coax cable can be feed through the whole antenna.

The other r/c antenna should be left mostly untouched. Only parts of the mounting base should be cut of. This makes it easier to solder the coax cable to it.

STEP 4: Assembling

Now you can feed the coax cable through the base antenna. In my case I needed to put some silicone oil on the cable because the tube was very tight.

Cut the outer insulation of the coax cable. Be careful not to cut through the shield. Put the cable through the brass nut. Splice the shielding and bend it backwards. Solder the shielding and the nut to the base of the r/c antenna.

Carefully cut the inner insulation of the coax cable and solder the core wire to the base of the other r/c antenna. Keep the cable length as short as possible.
Be careful not to connect the core wire to the shield. You can check your work with a multimeter.

Put some shrinking tubes over the construction and shrink it to fix the two r/c antennas in place.

Take the vacuum cup, cut a mounting slit into the back and press the antenna into it. You can remove the antenna from the vacuum cup any time you like. You can stick the antenna onto the window of your room, or car ... if you like. Don't stick it on a metal surface. You could use metal surface as a reflector, but the distance between the antenna and the reflector need to be calculated to make that work.

STEP 5: Usage

Finally you need to adjust your antenna to the frequency. On the picture you can see the antenna "tuned" to about 620Mhz. You can also "play" with the antenna and put the elements to different length. This could be useful if you like to "optimize" it for more than one frequency.
The antenna is build to be used vertically but you can also use it horizontally, if you like. The antenna diagram will be symmetric then. Keep that in mind while "pointing" to the broadcasting station.

Some links to sites about the sleeve antenna:
http://www.vdr-wiki.de/wiki/index.php/DVB-T_Antennen
http://www.freeradio.org/documents/Antenna_Basics.pdf
http://en.wikipedia.org/wiki/Coaxial_antenna
http://books.google.de/books?id=uxQP2Rfv44MC&pg=PT102&lpg=PT102&dq=sleeve+antenna&source=web&ots=GKMDTv4Ymy&sig=TbO0ToQvDWWCVB4Qwus3oHVcGOE&hl=en
http://emacademy.org/PIERS2k5zj/submit/get_abstract.php?status=valid&id=050110020348&pdffilename=050110020348.pdf

Comments

[electric guy]

Germany?

[У.Б]

Mongolia ??? 470-690

[brainzilla]

How about a 2$ version?
http://www.cnet.de/praxis/wochenend/41001557/die_beste_eigenbau_dvb_t_antenne_doppelquad_fuer_5_euro_basteln.htm
I build 3 of those today and the just work PERFECT.
100/-98% on three different TV-sets all over town.

You have to use their calculator with your region's max/min frequencies - this will give you the desired quad-side-length.

Make sure to make everything flat and rectangular, small dents are no problem.

They also have projects for multi-quads and an amplifier for about 3$.
(We all live in city center / like 3km away from the broadcast-antennas. If you are further away, you might have to use an amp.)

Also keep in mind, that this antenna has to turned into the right direction - although here it's enough to chose either the wall behind the TV or the 90° corner next to it.

[HaCo]

Hi, Your instructable inspired me to create an antenna in a different way. See the result here, but it's only for 482MHz.

[Derin]

You could also buy those antennas at electronics stores.

[Itsgoofytime]

In the USA, all TV broadcasting will be in a digital format, so no more antennas :)

[waterppk]

FYI, the digital TV will be transmitted on the same frequency spectrum, it's just that each channel will be compressed digitally to take less bandwidth, so you WILL NEED an antenna to get DTV in the future! The reason that TV's with analog tuners won't work (need the DTV tuner box to keep those working) is that they will tune themselves to the wrong frequency when you select a channel, hence the converter box takes the digital signal and then changes it back to analog frequencies and sends it along to the TV

[arubajack]

So let me ask you this....Is it possible to build a dtv converter box from scratch? I assume that you could easily do this with the right components to convert the digital back to analog in order to watch your outdated tv.

[frickelkram]

That is not as easy as you may think. Building it from scratch means much work in rf and also in digital technic. If you can find the cip sets neede it might work. I know that manufacturers like Atmel, DiBcom, Frontier Silicon, Siano Mobile and some others have chipsets ready to be used for multi standard digital tv receiption. Every solution I know off needs a microprocessor for the handling of the chip and the decoding of the digital signal. Telechips, for example, build processors based on ARM core that can decode the TV signals and also drive lcd displays. In any case you need to design a double (multi layer) pcb layout and be able to solder those small bga chips ...

[waterppk]

You can, what you'd be doing though is converting from the digital bandwidth (smaller than analog, many stations in the same frequency span as a single old analog channel) into the same frequency as a single old channel. The difficulty is that there is extra information transmitted on digital, so your converter would first have to identify the specific frequency band that the actual image and sound of the channel is on. It would then have to convert the digital image and audio stream and translate it into analog, which would have to be in the correct format and spectrum to be intrepreted by your analog TV. What a DTV box does now is transmits only on a single frequency (channel 3 or 4 in analog) and converts all the digital content to be rebroadcast in analog on that frequency. So it is the same thing as above, except that they're using a single analog channel, rather than rebroadcasting into all of the original analog spectrum, which would be difficult (expensive!) to modulate through all of those frequencies, and that is why they don't do it that way. It also allows the DTV tuner to only tune a single analog frequency, as if you tuned with the TV though all the analog spectrum, the DTV box would have to be constantly receiving and encoding all of the possible DTV channels into analog because it wouldn't know what channel you were on with the TV. Let me know if that doesn't make sense, I'd be happy to try to write more details if you're interested. Ultimately it would be very hard to build a tuner that could simulate all of the original analog spectrum, but otherwise what you would end up with was a standard DTV box with a variable analog frequency output (all analog channels, rather than just the three or four).

[Itsgoofytime]

Gotcha, I dont specialize in TV stuff, all I know is no more bunny ears :)

[technodude92]

analog, digital, lambda(wavelength), It's all Greek to me.

[ironsmiter]

actually, bunny ears work pretty well with the new converter boxes!

[Itsgoofytime]

That is at the end or beginning of 2009.

[technodude92]

Februrary 2009

[frickelkram]

DVB-T in Germany IS digital tv already. It is broad casted terrestrial and therefore you still need the antenna. In the country this sort of broadcasting is essential for the people. In the cities we got cable and do not need antennas anymore. I think that also in the US antennas will be needed in the future. ;-)

[dchall8]

You are misunderstanding what digital TV is. I live in a city but do not have cable or satellite at home, so my signals come in via antenna. I have a digital/analog hybrid TV and get both signals over the airwaves. There is a standard Radio Shack antenna located on my roof, wired with coax to the crawlspace under the house, and split to each TV and my computer. Having both analog and digital tuners available I can toggle back and forth between the two. As anyone with a similar setup knows, the digital difference is stunning. On analog TV David Letterman is wearing a gray suit. With digital he's wearing a gray suit with a red, white, and blue pin stripes. The other digital difference is that some channels broadcast several programs at the same time. For example our PBS station broadcasts 9.1, 9.2, 9.3, and 9.4. On analog I only receive what is broadcast on 9.1. With the digital tuner I get four different choices from the PBS archives and one is in Spanish. This digital multiplier effect took me from receiving about a dozen analog channels to more than 40 digital channels - all through the antenna.

[technodude92]

great 'ible. (+) if i ever get around to doing it i don't know if i have any digital stations where i live. one question though. Are all TV station transmitters polarized vertical?

[waterppk]

All omni-directional broadcasts are vertically polarized (TV, Radio, WIFI, etc) unless someone set them up incorrectly (it's just dependent on antenna set up).

You can check out more information here, search for "Vertical polarization is most commonly used when it is desired to radiate a radio signal in all directions over a short to medium range." if you are looking for just that.

[frickelkram]

As far as I know they are all vertically polarized. Btw. this simple antenna can be improved by adding a reflector. This way the gain of the antenna is higher, but then you have to point it in the direction of the broadcasting antenna to get the best results. Using a reflector comes close to building biquad antenna. This antenna type is popular for DIY wifi antennas. Look at this link: http://mdrc.org.au/Technical/ATV/quads/quads.html This guy built an antenna for a frequency range that is near to the one used for DVB-T. It should be possible to adapt this easily ...