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Thread: Ham Radio Antenna, a Simple Yagi

  1. #1

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    Default Ham Radio Antenna, a Simple Yagi

    A Ham Radio Antenna that works very well and can be made cheaply from materials sourced from local D.I.Y. store. This directional beam aerial is called a Yagi, it invented by a Japanese Scientist.

    An aerial depending on its design has not only gain and direction but it also has a property to reduce signal, this is called the front to back ratio.

    Ham Radio aerials are fun to build and they increase your power output leading to contacts over greater distances. Linear amplifiers are often used by Radio Hams to increase the distance of their transmissions, however a good multi element array increases your received signal too.

    Another property which is very useful when using a Yagi design is its capability to help you determine the direction a transmission is coming from, a technique employed during direction finding. Radio Hams often have fox hunts where they try to find a small powered transmitter hidden from view in a woodland area. By turning the boom of the Yagi to ninety degrees towards the direction of the transmission a deep null is experienced on the signal meter.

    A directional Ham Radio aerial including the Yagi design will also attenuate a signal if you point the back of the aerial towards the direction of the transmission. This is known as the front to back ratio and is very helpful when you wish to reduce an interfering signal.

    Many designs for the Yagi aerial have been published over the years, mainly to improve forward gain, free computer programs are available to optimise the design and efficiency and give the aerial builder tremendous scope for experimentation.

    Old "H" type Television aerials are often available at car boot sales or T.V. suppliers; they are cheaper to buy than new aluminium tubing. Using the formulae below you can make a Yagi to cover the air band or if you like listening to Maritime traffic on the V.H.F. band the Yagi would be a good design.

    A simple three element Yagi for the two metre band can be made from a short wooden boom. Stiff wire can be used to make the elements or the aluminium tube mentioned above.

    The Yagi consist of a driven element a reflector and a director.

    The driven element of any frequency you wish to design your Yagi to cover is calculated from this simple formula. Divide your design frequency into 468 feet; this length is the driven element.

    The reflector is five percent longer than the driven element; the director is five percent shorter than the driven element. Spacing between the elements can be between, point one, point one five or point two five of a wavelength at the design frequency.

    This spacing affects both gain and front to back ratio, some experimentation will needed to determine what is best for your design.

    The wavelength of your design is calculated by dividing the frequency into the speed of light. For example if we were making a Yagi to cover 150 Megahertz, we would divide 150 million into 300 million; our answer here is two metres. If you decide to use point two five of a wavelength for your spacing between the elements, your spacing will be 50 centimetres.

    Some experimentation with spacing and gauge of the wire will be required, but a good match to fifty ohm coaxial cable can be achieved.

    John Allsopp G4YDM http://www.qrz.com/db/G4YDM

    Article Source: http://EzineArticles.com/expert/John_Allsopp/1925417



    Article Source: http://EzineArticles.com/9550036

  2. #2

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    Hello John,

    When talking about Front to Back Antenna Signal Rejection, the only antenna I'm aware of is a 4 element LFA Yagi as it has 35 dB of Rejection off the back of the antenna. You would never hear any transmitted signal or interference off of that LFA antenna. Having a 35 dB Front to Back Ratio, an interfering station would need to increase their RF Power by 3162 times just to be heard and that's not going to happen from another Amateur Radio Station.

    BTW, I find it interesting the subjects you have chosen to write about here on the Ham Radio Forum and I hope the newest Amateur Radio Operators read your Subjects / Articles.

    Dan
    WA9WVX

  3. #3

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    John - when was the last time you found an old H type TV aerial at a boot sale? We've not had VHF TV since it closed down in the 70s! FM broadcast Yagis are probably the only source now, as the old TV VHF aerials are all probably corroded away to nothing now.

  4. #4

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    Quote Originally Posted by paulears View Post
    John - when was the last time you found an old H type TV aerial at a boot sale? We've not had VHF TV since it closed down in the 70s! FM broadcast Yagis are probably the only source now, as the old TV VHF aerials are all probably corroded away to nothing now.
    Hello Paulears,

    That's an interesting point you've stated about the VHF TV Channels in the United Kingdom and the Off-The-Air TV antennas. Our VHF TV Channels have moved up into the UHF Band here in the United States and the fact of the matter using discarded VHF Antennas is even worst as they aren't solid tube. They formed the aluminum tube but didn't weld it shut consequently there is NOT much strength using this type of tubing for 6 m, 2 m or 70 cm considering the heavy winds we've been getting. I've checked with a few home improvement centers and they don't carry 3/8" / .375 tubing in lengths of 1 m or longer lengths, only 36" which is a bit short for 6 & 2 m but okay for 70 cm for homebrewing Yagi antennas. It should be interesting to read John's reply.

    Dan
    WA9WVX

  5. #5

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    Dan - that's exactly the same as it was here - aluminium/aluminum (good different spellings) that had been presumably just folded and closed rather than extruded? Most older commercial TV and radio antenna designs seemed cheaply made like that. I wonder if his interesting beginners posts are actually old ones he's reusing? We really did have loads of two element TV VHF beams - 45-66MHz Our old channels 1-5, and then more at 180-210MHz (from memory) channels 6-12. This meant that as the new commercial stations popped up you needed two antennas. The H type for BBC and a 7 or so element for the higher VHF ITV channels. In 1968 we started UHF broadcasting in the modern channel 21-69 range. This is what makes me think John's info needs a little modernising. I remember asking customers ordering new antenna systems if they wanted the old H type removing? That was just before I got my ham license in 1980.

  6. #6

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    Hello Paulears,

    I seem to remember back in the 1950s & 1960s that we had a lot of folded dipole antennas built with Aluminum (USA) tubing and they were cut / tuned for Channel 2 through 13 mounted with lower channels / elements on the bottom of the mast pipe and the higher channels / elements mounted about 1' / 1/3 of a m above the lower elements. Of course everyone used 300 Ohm twinlead but many technicians / installers didn't provide the proper twisting of the cable to eliminate multipath signal interference and it could get bad if your home was a good distance away from the TV transmitters. I was a young whipper snapper back those days. I started into Short Wave Listening at the age of 12, build a Knight Kit Receiver, my father installed an "L" antenna and I was off in the running trying to learn as much as I could about ham radio.

    Fortunately I had an "ELMER" who lived a half block from our home and I continued to learn. There's no way I could have learned as much as I did without this guys help. He administrated my Novice test in August, 1968, he looked over that written exam and I had missed one question. He had tuned in a station 15 m CW and made me copy everything, as I was writing it down, it didn't make too much sense to me but my ELMER tuned in a KP4 Puerto Rico station -- no wonder! Then my ELMER handed me the Technician test (which was the General Theory) and I hadn't studied for that license by I gave it my best shot, failing only two questions. I didn't receive my licenses to October, 1968 and by that time I was going to Trade School learning Electronics for my career in Radio Communications.

    I had redirected myself as for upgrading my ham radio license and had a new goal to reach which was the First Class Radiotelephone Operators License needed to perform maintenance on Radio & TV Stations, the two-way radio communications industry (out in the field), Citizens Band & Amateur Radio equipment performed in a Dealers Shop. It took me a couple of years to pass those exams and Second Class License was much tougher than the First Class License exam. I've held and used my First Class License since August, 1971, providing Bread & Butter and a whole lot more during my career at Motorola, Inc. in the Land Mobile Two-Way Radio Industry. Looking back on my career, I had so many different positions that most people would have envied me if they were working in the same field and having the Amateur Radio License provided a two-way street for learning every new technology a bit quicker as what I learned in my career I could apply to my hobby and what I learned in my hobby I could apply in my career.

    When I was let go in June, 2004, I had moved up to a Non-degreed Research & Development Engineer for the Infrastructure (Base Stations & Ancillary Product Lines) Specialty Products. I worked with the older 12 Kb and the newest 128 Kb Encryption / Decryption equipment, OTAR, Export Control Assignments, over saw the last hardware SP Build through the Factory, Wrote SP Manuals, Scripted Wildcards for special applications, Performed the SP Bid & Quotes for the End Users, the Sales Personnel, System Engineers, worked with the End Users all the way up to the CEO of Motorola, Inc. and Performed Maintenance Of the Line assisting the Factory's Technicians when they encounter problems. There wasn't a dull moment in that job position. I had gained the respect from my peers in Engineering and often I would touch base with those people on questions / answers on some of the Products.

    73,

    Dan
    WA9WVX

  7. #7

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    Dan - I didn't know 300Ohm was the standard, stateside - in the UK everything was 75Ohm coax. The need to cover the lowland and highland broadcast aerials meant loads of diplexers up top, as everyone needed both down the cable. In the 70s, we started to get lots of Japanese hifi tuners, and these always had 300Ohm terminals, which many of us were confused by - just something unfamiliar. Then it became standard to include a flexible dipole made from ribbon. Of course, we still carried on with coax to our FM broadcast outside antenna systems. When I became a ham in 79/80 I discovered the other uses for 300Ohm balanced cable, but until then it was magic. The hams were making 2m aerials from the stuff of course. I just never knew it was used for TV your side. Since this topic, I've been looking locally and I can't find a single home with old VHF TV aerials up. Pointless really as they'd have crumbled away years ago.

    Your career (work and radio) sounds a good one. I originally wanted to work in broadcast radio, and failed my interview at the BBC because at 18 I didn't understand bias current in audio recorders. I thought I did, but I didn't. When I qualified I spent two weeks in a small radio workshop and found none of the long term guys liked radio at all as a hobby - so I started selling the new fangled U Matic video recorders to businesses - which cost more than I got paid in a year. I'v done loads of radio related jobs over the years, but always in strange circumstances. Training radio users in civil defence when we didn't like the Russians, then marine and business radio - a bit of aviation - but in most cases radio was always a number two. I've spent most of my career in theatre and events, and with equipment hire very important to us, radios are always there somewhere. Broadcast links, comms links, and generally people running around with radios doing the work. We have various licenses to use, fix, hire and install all kinds of radio stuff - so we're primarily users - so along the way we have to cover practically anything. Digital is making us scratch our heads a bit at the moment. Thanks for sharing the US history - nice to hear how it's similar, but different!

  8. #8

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    Thank you Dan, always great to hear feedback, where to pitch a blog is often the hard bit hihi...73 have a good christmas...John G4YDM

  9. #9
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    This guy has a heck of a site with all of the math and voodoo already hashed out for building Yag's.
    http://www.k7mem.com/Electronic_Note.../yagi_vhf.html

    I've got one I built from his measurements sitting in my garage right now waiting on an expedition. Pretty good in all of the tests I've used it for, now I just need to drag it out into the wilderness! The one I built was a 6 or 7 element and had something like 8.x gain according to his math. I'm not sure how to calculate gain in the field or if you actually notice the difference between 7.5 and 8.2 or whatever.

    Since he's in Northern AZ, I've E-Mailed back and forth with him and talked with him on a linked repeater system. Great guy. Really gets excited when he hears you are using his plans and he's super helpful. If you are building one, I recommend giving Martin a call. Tell him KG7NDC sent you. He might not remember me (it's been a few months) but then again he might.
    N5MKH - The only thing that separates man from animal is our affinity for toilet paper. Once we as a society lose that affinity we begin to descend back into the animal kingdom, and after three or more days you will find the food chain beginning to invert on itself. Up is down and down is us and man is no longer an alpha predator.

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