The balun piece does not have to be a U shape as shown in Figure 4; it can instead be coiled, but not too tight! Forcing the coax into a tight coil can affect its characteristic impedance, throwing off the impedance transformation. The box I used is smaller than it should be, but not too much smaller. Call it paranoia, but it occurred to me that the screws used to hold the box together might interfere with reception. I ditched them and used two large zip-ties to simultaneously hold the box shut and fasten it to the boom.
Dipole location on the boom is critical to reception. Not wanting it to move around, I used a little hot-glue to fix the box supporting the dipole to the boom.
I also used a little dab on each element just to keep them from moving. The elements themselves could slide if the fit is loose, and the folded dipole should be in a plane perpendicular to the boom and not able to rotate. Another objective is to keep the dipole centred both vertically and horizontally on the boom axis. See image at top for an front view of the antenna. One additional problem: whereas conventional coaxial cable used plated copper for the woven shield in the coaxial cable, newer coaxial cable uses plated Aluminium.
I ended up using a household wiring marrette to hold the shields from the two ends of the balun and the incoming coaxial cable together; not the prettiest solution, but after 30 min of trying to get solder to wet Aluminium, and wrecking several pieces of coax, one looks for alternatives.
Once the antenna was finished, how to mount it? Generally the higher up above obstacles the better, so a roof-top mount is a good idea.
For initial testing, I clamped the boom behind the reflector in a bicycle stand. This turned out to be very convenient. Once it was confirmed working; i. The probability of needing it to repair a bicycle was, and is, quite high. To mount the antenna on the roof, a J-bracket for mounting a satellite dish was used.
The bracket was is fastened to a roof peak, and to the bracket was fastened a 2 metre length of 1. This happened to slip right inside the J-bracket. It was purchased from a metal supermarket, and was much less expensive than expected. Figure 5. Need four U-clamps: Two in 1. McMaster Carr is a great source for all things U-clamp. Orientation will doubtlessly come up. For television, the elements should be horizontal; for FM radio, vertical. The reason for this is that RF signals are polarized; the angle depends on the orientation of the transmitting antenna.
The orientation of the receiving antenna should match the transmitter. The antenna will still work with the orientation wrong, just not as well. One last mounting issue is grounding. Putting an antenna on a rooftop and connecting it to a receiver brings the possibility of a lightning strike wiping out the receiving equipment. If you follow this blog post, build and install an antenna on your roof, you do so at your risk , not mine!
I am not responsible for you not hooking up your antenna safely. Residences have a rod driven into the ground and clamped to the metal conduit in the building. Usually the rod can be found sticking out of the ground where power enters the building, near the electrical meter. A ground block for coax has two female coaxial connectors, but also includes a screw terminal for grounding the shield on said coax. Use a heavy ground wire here, do not skimp. Figure 6. These pictures were taken a little while after construction, and a patina is clearly already forming.
This should not affect performance. I thought about plating or coating the copper with something so as to preserve its shine, but other than looking pretty, in itself a debatable point, there is no benefit.
Note the marrette above. Since the Aluminium ground shields will not wet with solder, the marrette makes an easy alternative, even if it does look a little clunky. Figure 8. The mounting bracket is shown above. It would sure be nice to extend the range when off-road.
Cell phones use much higher frequencies than television, which translates to a much smaller antenna. The biggest obstacle is the disappearance of antennas on the outside of cell-phones. Does there need to be? Why not make a Yagi with just the parasitic and reflective elements, but no dipole or balun, cut for cell-phone frequencies.
This is much simpler because there are no cables, balun or folded dipole to worry about. The most obvious thing is wifi. Wifi is mostly in the MHz band, but some standards support 5GHz, which means a smaller antenna than even the cell-phone version.
This would require different coax, and SMA connectors, and could really extend the range of wifi for certain devices. Seems like external antenna connectors are disappearing from this environment too; could be smart to do something like the gapped-boom idea, then just slip the side of a tablet into it.
Your email address will not be published. Nice article,plan on making four stack h box array,thanks for your good work,More fun to make,than watch tv. This is one of the best tutorials about building a yagi antenna i have seen. Also great illustrations for the yagi elements. In this example, is the coax cable soldered directly onto the copper pipe from the driven element? I used a dremel tool to drill small holes through the ends of the driven element; two holes at one end, one at the other.
I then inserted the centre conductors into each hole so they passed right through and could be soldered on both sides of the copper pipe. This gave it quite a bit of strength.
The shield from both ends of the balun and from the feed line are joined together with a marrette. I would have preferred to solder them together, however the shield on this cable is Aluminium, so it does not take solder. All you need to do is put some light oil on the aluminium, scrape through that with a knife, and the oil will stop the oxidation just long enough for you to solder to it.
Give it a try! Did you get them to work as hoped? Kevin in Waterford. So, I built one using bowtie dipoles and the design works really well. Thanks for the insights. Currently, I am considering adding reflectors behind the dipoles to really pump up the reception.
Regarding your not plating or finishing the copper… This has been a nagging concern for me. Do you think dipole oxidation might reduce reception?
Glad you liked it, thank-you. With respect to the choice of cable, this is what was to-hand. If you need multi-directional reception, you can always use more than one antenna. It gets a little tricky because they can affect each other negatively if connected to the same piece of coax.
I used a conventional splitter to join a North-facing to a South-facing antenna. Another alternative is to have one antenna for each market and bring them each into separate tuners.
Terrific explanation of the balun! I am building an antenna to use in Port Moody and am wondering how much gain I need to design for. During the Olympics I was able to watch it on digital tv on rabbit ears, no problem. I double checked all cables for continuity, was careful about impedance, etc. No go. I am basically at sea level in Port Moody on the block on Clarke Street, about a kilometre and a half from Rocky Point. I pointed antennas to Mt Seymour. With another tool I could measure signal where channel 22 is supposed to be.
But no luck with picture. I wonder if the new sky train and other development has brought into play new RF that is desensitizing my receiver? Seymour under 11km. If there is a train passing by, there could be some spurious noise its motors, but it would need to get close to your antenna.
That is, the signal is strong enough to overwhelm your tuner? You could try just rotating your antenna away from Mt. Seymour, and see what happens as you get further off of optimal. If that proves to be the problem, an attenuator in your feed line will settle things down.
This article was awesome. Those towers were miles away. The Fox channel, however, is in Hardin Montana, more than 75 miles away and in a direction that was more obscured by a ridge.
The commercially made Yagi could detect the signal, but it was too weak to get a picture. This article provided the practical help I needed to actually construct it. The antenna works great for the and MHz stations! Thanks for your help! Another option might be scribing your circle on the corners by turning the mounting square 45 degrees. This way you can find the top edge of the pipe by moving it slightly until the drill just touches. I would think any good conductor will work.
Others have commented here that Aluminium can be soldered so long the surface is scraped clean of oxide and immediately covered over with a flux of some kind an oxide layer on exposed Aluminium will form super fast. Since you only have to solder to the ends of the folded dipole, you could use copper for that, and whatever metal is available for the reflector and director elements.
I used an inexpensive irrigation pipe. Great article! I have a question regarding the dipole. Can I use a single dipole or does it have to be folded? I live 12km from London Heathrow airport clear line-of-site to my location, thanks to the LHR tower but want to build a yagi for my Baofeng to listen to the The VK5DJ tool is amazing but it seems to push the constructor to use a folded dipole.
Can I use a simple dipole mounted with perspex using the calculations from the tool? I guess the single dipole will have less gain? This is killing two birds with one stone, as both a balanced to unbalanced conversion and an impedance transformation are being accomplished simultaneously.
Excellent response; many thanks for the quick reply. I appreciate your answer, which has really helped. I made your yagi cut for mHz PBS ch This antenna is attic mounted and a cable splitter is used as a combiner, then a low noise Weingard preamp before distributing to my three household TVs. I will be fully cutting the cord from FIOS in another 14 months. Nicely done Craig! I have mentioned previously that I have this antenna combined with a second antenna facing the opposite direction for dual market reception.
I suggested routing the antennas to separate tuners as an alternative to merging them with a splitter, but I like the idea of a pre-amp too. Hmm… maybe I should explore a multi-input UHF amplifier design. Congratulations on one of the cleanest designs and best write-ups on the internet. Beautifully done.
When you combine two antennas facing opposite directions, you are in effect reducing the gain of your array to 3 dB less than a single antenna. Doubling the complexity for half the gain is not the optimal approach. There are various ways to explain this without resorting to heavy duty math. Perhaps the easiest to understand is that each antenna contributes noise. In combining two antennas facing opposing directions, you double the noise, but not the signal. The fallacy with this argument has to do with the fact that the signals add coherently, but KTB noise does not.
For a bidirectional array, you would do better to build a broadside phased array. You can achieve the same gain with one quarter the number of elements. A reflectorless planer bow tie array would be brilliant for the job.
Thank you for the way you explained not only how to build the antenna, but the theory behind it. A ohm matching transformer is a balun designed to accomplish the same impedance transformation, so yep. One caveat though: the coax balun is more robust than the transformer balun, and probably allows more signal to reach the tuner.
Stated another way, I suspect the insertion loss of the transformer is higher than that of the length of coax, and being of more delicate wire, more prone to failure. That type of balun is usually meant for systems where there is a very strong signal available from a cable provider, so a little loss is not a problem. Also, those little transformers are usually intended for indoor use. Excellent article! What options are available to evaluate the the antenna once it is finished?
I have heard of expensive antenna analyzer equipment but is there something less expensive options? Would it be possible to use the tv broadcast as the signal source and then use for instance an USB dongle such as rtl-sdr as the receiver.
Or is there some other better method? Thank you! When I built the first one of these, I was not aware of the rtl-sdr dongles for radio scanning. If you opt for the rtl-sdr route, you might find the results a little confusing because there will always be some multipath interference caused by reflection off trees, buildings, etc. You may end up pointing the antenna at a building to pick up the best signal. What I did to get it right, was connect the antenna directly to the digital tuner in a PC running linux.
I then took a laptop up to the antenna -the roof- and opened a terminal ssh session into the linux box with tuner. After that, I could rotate the antenna and immediately observe the effect on signal strength. When I thought I had it optimized, I would walk several meters away from the antenna and recheck the strength on the laptop.
You do need to get away from the antenna for a final check, as your own body will influence reception. You generally need a dish with an lnb for satellite reception. Great article with insight. How different is the commercial 75 ohm to ohm adapter from the balun created for the yagi?
Would it be a usable subtitute? Fico muito Fraterno do Meu Oriente. Being a newbie to antennas it will take some time to absorb all this info. In the meantime, if you could help me with this problem. Any help would greatly appreciated Jess. I am trying to find out how to design a multi-channel antenna. Do I need to have a driven element for each channel?
Can I just make a yagi with multiple driven elements? Also, can a longer director element of multiple full wavelengths compensate for directors? Can a longer driven element of multiple full wavelengths compensate for directors? Interesting question. This is a little stream-of-consciousness, so bear with me:. The directors and reflectors make a Yagi very directional, and the driven element, in this case, receives that highly directional high gain signal from one direction primarily.
The trouble is, you want the folded dipole to resonate at a harmonic of its fundamental. It may work, but I suspect very poorly as compared to a folded dipole cut to the wavelength of the actual frequency you want to receive; also, it will be really exceptional at receiving its fundamental, and the passive elements will contribute very little, so it will have very wide lobes facing forward and backward on the axis of the boom. In terms of impedance, the desired UHF frequency will be very poorly matched to the antenna.
I had a bit of a glitch with the website recently, and your last question, asked May 11, was lost. Now, I have another question about television antennas. Specifically, the folded dipole as it is arranged relative to the boom. What I see is: Some Yagis put the parasitic element in the middle of the folded dipole plane.
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