Introduction: E.T. - UHF Indoor TV Antenna
If you can’t use proper outdoor TV antenna you’re most likely stuck with “rabbit ears”. They use small, built in loop antenna to receive UHF broadcasts, while telescopic rods are only used to receive VHF broadcasts. Most of the digital terrestrial television channels in Poland are now broadcasted in the UHF band so just this small loop is actually used. If you try to receive weak signal (for example large concrete structures are blocking it) with this loop antenna, it won’t work very well.
That was the situation in my grandma’s apartment. I decided to built new antenna from simple household items. It was probably more expensive and definitely more time consuming than just buying a new antenna. But then, Jedi Knights built their lightsabers, instead of ordering mass-produced ones at an online retailer that owns drone-operated warehouse half a galaxy away.
Antenna working in such conditions needs to be directional (but not too much) and wideband. I chose to use bow-tie dipole, corner reflector and Roberts balun. Antenna is designed to be attached to a window sill with a hook-and-loop fastener.
E.T. stands for Electromagnetic Trombone. Distance between apex of corner reflector and bow-tie dipole can be adjusted by the means of a slide mechanism, hence resemblance of the antenna to a trombone. Change of this distance changes impedance of the antenna for various wavelengths, as can be seen in Fig. 2 here.
Step 1: Parts and Tools
Parts and materials:
- around 2 m^2 of cardboard (single wall corrugated board), 72.5x152.5cm and 67.4x112.4cm pieces should be fine
- aluminum or copper foil
- around 2m of 75Ω coaxial cable and a male Belling-Lee (Europe and Australia) / F-type (rest of the world) connector [I used male to female TV aerial cable]
- 0.5mm diameter copper wires (you can extract them from UTP cables)
- small piece (16x5mm) of 0.8mm thick PCB with copper removed
- electrically conductive adhesive (you can use solder and soldering gun if you chose to make bow-tie dipole out of copper foil)
- craft glue or basically any glue that will adhere to paper
- insulating tape
- scotch tape
- small nail (I used one that is 3cm long and 0.6mm in diameter)
- 4x clothespin
- strong self adhesive hook and loop tape or relatively small object that weights at least 1kg (I used combination of lighter, 0.5kg object and two strips of weak tape)
- diagonal cutter
- 2x pliers
- utility knife
- set square
- soldering station
- 1mm drill bit
- drill press or rotary tool
- rubbing alcohol and paper towels
Step 2: Cutting Out the Cardboard Parts
Use pencil, ruler and set square to mark edges of parts from cardboard_revised.pdf on corrugated board. Draw lines that will be used to position rail parts G and H onto parts L. Then cut the parts with an utility knife or scissors.
EDIT: Previous file cardboard.pdf contained wrong dimensions for whole 72.5x152.5cm sheet and 15cm dimension of part J was missing.
Step 3: Attaching Reflector
Fold the foldable edges of part B. Glue aluminum/copper foil to 42.4cm long sections of part B. You may use a leftover piece of cardboard to spread a glue. If your aluminum foil is less than 30cm wide, use small rectangular pieces of foil that are few cm wider than empty space to cover rest of the 42.4cm long sections of part B, then use scotch tape to secure part of small foil piece that doesn’t make contact witch glue in place. After that, attach firmly external edges of reflector to cardboard, also with scotch tape.
Step 4: Connecting Cardboard Parts Together
Fold the foldable edges of parts L then cover flaps with glue and press those flaps to the back of part B. Glue parts H to part L, while making sure that part F can freely move between two parts H. Then, glue parts G on top of parts H, while making sure that 10.5cm wide flap of part D can freely move between two parts G. When this is done, you may place horizontal strips of scotch tape below and above track parts G and H to prevent whole construction from expanding too much to the sides.
Step 5: Making Bow-tie Dipole
I based design of antenna’s driven element on this Bowtie Antenna Designer, calculated for center frequency of 600MHz, as UHF TV channels are broadcasted in range of 490-706MHz at the nearest transmitter. Designer calculated that I need dipole that has a width of 187.4mm, height of 124.9mm and distance between pieces of 10.3mm. I rounded those numbers to 188mm, 125mm and 12mm. While drawing edges of bow-tie on part D, I assumed (most likely incorrectly, but this mistake will just shift center frequency closer to 0) that I need width of each bow-tie piece to be 94mm. I covered area inside edges with glue and placed the foil on top of the adhesive. I folded foil along the edges and then cut along newly created creases with knife.
Step 6: Connecting Supports of Bow-tie
When glue holding parts G and H is dry, slide part F inside those tack parts and mark with a pencil where edge of part G is in different positions of part F. Fold flaps of parts C and D, then glue 5x5cm flaps on ends of parts C together. Next, glue parts C to the back of part D. After adhesive has dried, mark with a pencil lines that will be used to position parts A. On the lower C part mark single line in the center, while on higher part C mark two lines, each 1.5cm apart from center. When this is done, attach parts A, so that center section of part A is on the line and flaps are glued to part C. Glue parts F to both ends of newly formed cardboard construction, while making sure that flaps stay inside pencil marks on part F. Then, after waiting some time, you should check that whole construction can slide in tracks.
Step 7: Making Roberts Balun
As I was using TV aerial cable with plugs on both sides already installed, I cut one of them, so only a male Belling-Lee was left. Then I started to remove insulation and shield from exposed part of coaxial cable with utility knife, to the dimensions from balun.pdf (5mm of shield is visible, 3mm of inner dielectric is visible and at least 10mm of center conductor is visible). When this was done, I’ve cut the cable 100.2mm behind start of the shield. Later, I made 11.2mm long diagonal cut on the newly formed end of short piece of coaxial cable. I drilled a hole with 1mm drill bit 68.5mm from the start of the shield through center conductor and removed its shorter part with pliers while keeping longer part in place with a second pair of pliers. Then, I removed 6.4mm long section of outer plastic sheath 76.2mm from the start of the shield. Next, I took care of longer piece of coaxial cable and treated exposed end the same way as on the other piece and also removed 6.4mm long section of outer plastic sheath 76.2mm from the start of the shield.
When both pieces were ready, I pressed two cable pieces together (longer “edge” [I should actually say “element of the truncated cylindrical surface”] of shorter piece facing longer piece) and joined 6.4mm long sections with 0.5mm diameter copper wires wrapped around and soldered together (unfortunately 5mm in diameter cable that I was using had aluminum shield, so wires are just tightly wrapped around the shields, not directly soldered to them). Later, I attached few centimeters long copper wires to 5mm sections of uncovered shields. Next, I reinforced connection between two pieces with small amount of insulation tape and placed 16x5mm 0.8mm thick PCB (copper removed) between 5mm long sections of shields (holding it in place with tape). To finish balun, I soldered exposed center conductors together.
Step 8: Connecting Bow-tie Dipole to Roberts Balun
Punch with small nail two holes (around 8mm apart from each other) almost through center of part D (they should be located near vertices of bow-tie pieces), between upper parts A. Clean bow-tie pieces surface with paper towels soaked in rubbing alcohol. Pull copper wires through those holes.
Secure balun in place with insulation tape attached to it and cardboard parts A and C. Angle the wires, so they go through bow-tie pieces vertices and when in front of the foil they are horizontal. Then, hold center parts of wires with a strip of insulation tape, do the same to the outermost parts of the wire. I was attaching short strips at first and then sticking more tape to them so that tape goes beyond edge of part D.
When wires were secured in place, I applied layer of electrically conductive adhesive between foil and wires and when adhesive was dry I applied another layer.
If you used copper foil you may simply solder wires to copper.
Step 9: Pulling Coaxial Cable Through Reflector
Cover small portion of reflector’s foil most center part with scotch tape to reinforce it (use at least few horizontal and vertical strips). Then, make vertical cut through reflector’s apex, from the cardboard side. Next, make horizontal cut from the foil side that will intersect vertical cut at the reflector’s center. When this is done, pull the cable end with the plug through newly made opening in the reflector (if your cable didn’t have plug already installed you may cut smaller opening and install the plug when the cable already goes through the reflector). Then, slide bow-tie supporting structure into tracks and pull enough cable out of antenna so that cable between balun and reflector is straight. Antenna should be now operational, so you can connect it to TV and see if it works. You may need to position bow-tie dipole farther or closer to the reflector if you have problems with reception.
Step 10: Constructing Stand for the Antenna
Fold flaps on parts J and slide them into each other so that flaps form two surfaces to which parts E can be glued. When this is done, join pair of parts E with glue, so that fluting on both parts E is perpendicular. Next, repeat this for two remaining parts E. Later, glue sets of parts E to upper and lower flap surfaces of parts J. I used few pieces of scotch tape to secure parts in place during glue drying.
Later, fold flaps on part K and glue wider one to part B (so it points downwards) and shorter ones to flaps of parts L (so it points upwards). Foldable edges of part K should be located on top of reflector’s apex and support coaxial cable. Again, scotch tape should be used to secure parts in place while glue is drying. Then, reinforce main surface of part K by gluing to it part I, and do the same to 15cm wide surface of part B (15cm wide surfaces of parts B and K should be able to make contact witch each other). Now you can join those 15cm wide surfaces with a pair of clothespins (each of them located at the opposite side, ideally one of them should be higher than the other).
Attach “massive” object (in my case it is rectangular cuboid made out of cardboard containing 0.5kg of scrap metal) to the side of the stand, at the top of bottom parts E, with scotch tape or glue. Below, on other side of parts E set, attach self adhesive loop tape (I used two strips of 20mm wide tape placed parallel to each other and to one of part’s E edges). I covered ends of those strips with scotch tape, so that they won’t detach from cardboard. Next to the loop tape, I attached few layers of cardboard with scotch tape so that upper surface of antenna’s stand will be level and slope of a window sill will be negated. I attached longer strips of self adhesive hook tape to the window sill.
In the center of 30x30cm section of part B, on both sides, apply decent amount of scotch tape. Do the same to the center of the stand’s top part E (just from the one side of this part E, do no tear anything). When this is done, put main part of the antenna on top of the stand and punch small nail through all tape layers, right at the center of 30x30cm rectangles.
If your stand is attached to a window sill already, you can now rotate the whole construction around the nail. When you found the right position, secure antenna in place by attaching two clothespins to the edges of 30x30cm surfaces on top of the stand.
On some of the pictures you may see some additional strips of tape that mitigate little quirks of cardboard construction made by my hands (you will definitely do a better job). One particularly worth mentioning is one located just below the apex of reflector and prevents part K from folding.
I decorated the antenna with commercials taken from page 108 of February 1954 issue of Radio & Television News . I did it because Radio Repair Man character from ERIE ad looks very much like something that might have been inspiration for Vault Boy from Fallout game series.
Step 11: (optional) Grounding the Reflector
To ground the reflector I used shielded power cable with inner wires removed. Ends of remaining shield were twisted and insulation tape wrapped around places where shield comes out of plastic jacket. Two holes going through the reflector (and cardboard supporting it) were made. One end of shield went through both holes and was twisted (on the cardboard side), so that this shield end was firmly pressed against metal foil. Later, on the foil side, shield end was covered in layer of insulation tape, additionally pressing it against the foil. On the other side more insulation and scotch tape was added, in the quantity sufficient to firmly attach cable to a back of the antenna. Shield from the other side of the cable was connected to a well grounded metal surface (in my case it was a radiator, shield was kept in place by the insulation tape). I actually did not notice any difference in operation of the antenna that was grounded and that was not, but grounding of antenna’s reflector is always advised.
WARNING: If you were to build the antenna out of sturdier materials and place it outdoors, proper grounding of the antenna is paramount. Copper wire that is at least 2.5mm in diameter should be used to connect antenna’s reflector to the ground and more reliable method of making contact between wire and other conducting surfaces is to be implemented.
Step 12: (optional) Installing Signal Amplifier
Coaxial cable was cut and between newly made cable ends a signal amplifier was installed. The cut was at such a distance from balun, that bow-tie dipole could be placed completely in front of the antenna while the signal amplifier is still behind the reflector. Male F-type connectors were installed on both of the cable ends. To install this type of connector you need to remove few centimeters of outer plastic sheath, angle braided strands so that they now encircle a portion of a remaining plastic jacket and then, remove almost all of visible foil and dielectric (remaining portion should be as long as a smallest diameter section of the connector). Then, if you have a twist-on connector, you should screw it onto braided strands and later, trim any strands that come out of the connector. Central conductor should be cut so it extend a little bit from the connector. As mine connector was slightly too big for my 5mm diameter coaxial cable, I later also wrapped some insulation tape around place where cable starts to come out of the connector, to make sure that connector won’t fall off the cable.
Signal amplifier that I purchased had a male connector on antenna side so I needed to use female to female barrel connector coupler to install amplifier. Amplifier is powered from the TV side, so I replaced original Belling-Lee connector with a special one, that was attached by two wires to a 12V power supply. I removed connector’s cover and loosened three screws . To attach coaxial cable to it, I removed portions of its layers so that a visible portion of the center conductor was as long as long was metal part designed to hold it, a visible portion of dielectric was as long as long was the gap between metal parts and a visible portion of the shield was as long as long was metal part designed to hold it. Braided strands were twisted together into one rope-like structure that ended exactly were foil did and was parallel to the center conductor. Those strands were placed beneath metal plate that was held by two screws, while center conductor was inserted between another metal part and a square nut located inside this metal part. Later, all three screws were tightened and connector’s cover was put back in its place. Next, male connector of antenna was plugged into female connector of TV set and power supply into 230V AC. Signal amplifier was now operational.
Unfortunately, particular amplifier that I used was too strong. To remedy this, I installed simple electronic circuit that lowered voltage powering amplifier (it should be done with power supply disconnected from the mains). This circuit was based on 3.3V Zener diodes that could be connected in series with power supply. To make a base for physical construction two SPDT (SPST switches can be used as well) toggle switches were joined together by wrapping insulation tape around them (shorter sides were in contact witch each other). Then, to each switch a 1N4728A Zener diode was soldered (so that switch could short circuit the diode). Later, those diodes were joined by short piece of wire and a BAT48 Schottky diode was soldered in series witch them. Next, an angled anode of 10 uF electrolytic capacitor was soldered to the cathode of BAT48 and piece of wire that was few centimeters long was soldered to an angled cathode of that capacitor. When this was done, power supply wires coming out of the special connector were cut near this connector and insulation striped near newly created ends. On the connector side positive wire (you can identify which one is it with multimeter or by looking for a white strip on the wire insulation) was connected to the anode of capacitor and negative wire to the cathode of the same capacitor. On the power supply side, positive wire was connected to a cathode (one that until this point wasn’t connected to anything else than the switch) of the Zener diode and negative wire was soldered to the wire coming out of the capacitor’s cathode. Later, this whole circuit was wrapped in insulation tape (except Zener diodes, so they can dissipate heat more easily and so that it can be seen to which leads of switches they are soldered). Now, when Zener diode is not short circuited by the switch, voltage powering amplifier is lowered by 3.3V and that in turn reduces amplification.