View Full Version : 2 meter j-beam
06-27-2003, 04:46 PM
This weekend, I am going to build a j-beam, that will attach to my existing copper j-pole. Plans are from a QST article, xeroxed for me, I do not have the issue date.
Now, which way do I point the thing? Using a rotator, if I want to point NorthEast, do I point the directors NorthEast or the reflector NorthEast? Alan KG4WOA
06-27-2003, 04:50 PM
The reflector "reflects" the signal towards the director, so you point the director in the direction you want the signal to go (the shorter end).
06-27-2003, 05:41 PM
Unfortunately, what some people are now calling a "J-Beam" is NOT a J-Beam! Adding a reflector and a director (or directors) to a J-Pole antenna does NOT make a J-Beam!
The J-Beam was developed primarily in the United Kingdom during the 1950s and consists of two yagis (you can stack more than one J-Beam) that share a common, skeleton slot driven element. Although many of these antennas were "home brewed", they were also produced commercially in the UK and imported into the United States by Gain, Inc., of Chicago, Illinois.
These antennas have good gain, front-to-back ratio, and are relatively broad-banded in terms of frequency response. They were very popular in the United States during the 1950s and 1960s. Construction details were given in the RSGB (Radio Society of Great Britain) VHF Handbook for a number of editions.
I built one back in the late 1960s and it worked very well. The one that I built had a total of 8 elements on each yagi section.
The antenna consists basically of sections of 2 stacked yagis in each section with the driven element being of the skeleton slot design. This puts a 1/4 wave length of tubing on each yagi with, if I remember correctly, a 1 wavelength vertical section on each side of the driven element. The feedpoint is in the middle of the vertical section. Of course, this is when the antenna is in horizontal polarization. For vertical polarization the 1 wavelength section goes horizontally and the elements of the yagis are in the vertical plane.
Thus, since some people are now calling the addition of parasitic elements to the J-Pole a "J-Beam", confusion is going to result! The original J-Beam does not resemble what is now being called a "J-Beam" except that both use yagi type parasitic elements.
As a result, I think that some other name needs to be used for the newer designed antenna since "J-Beam" (or actually "J" Beam as was the original trademark) means a completely different beam design.
Glen jumped in there with a very good clarification of terms about the new vs. the traditional names for these antennas, before I finished my posting, but I am going to stick with the information I am sending you, based on the newer, common use of the term, whether the usage is absolutely correct or not.
Make sure you understand that the "reflectors" are the longer elements, and the "directors" are the shorter ones ! UDX's comments might be misunderstood if the beam you are making has several directors ahead of the driven element ! That would make the "boom" longer, but the elements would still be about 5% shorter than the driven element ! I am sure he is referring to the length of the elements, themselves, and not necessarily how much boom is sticking out in either direction from the JPole driven element! Make sense ?
Hey, while I'm thinking about it, remember that to work repeaters, you should use the beam in the vertical orientation. I would guess that is why you are building a JPole driven beam. That type of antenna just begs to be used in the vertical polarization!
VHF/UHF SSB, on the other hand, is almost aways done in horizontal polarization. You will see almost a 20 db drop in signal strength if you are crossed polarized.
73 and hope to hear the results of your project soon!
Glen is correct about the term "J" Beam.
B Sykes, G2HCG is the designer of the "J" Beam using the "skeleton slot" driven element design (this design can be found in the RSGB VHF-UHF Handbook, 1st edition)
In 1992, G2HCG retired and sold his company. #Unfortunately this original antenna design was eventually dropped by the last owner - South Midlands Communications (SMC) Ltd. #Eastleigh, Hampshire. SO53 4BY England. #Spectrum International, the last US distributor, sold their remaining inventory at the 1993 Dayton hamfest. #
I just received an e-mail from Mike Bennett, Sales Manager at SMC, informing me that they have left the amatuer radio business and have disposed of the J Beam parts and designs. [Never building the antenna after Mr. Sykes retirement].
I recently sent a letter to B. Sykes inquirying if copies of his original design documents still exists (2 m and 70 cm antennas).
If NOT, then I will need a acquire an existing J beam antenna for reverse engineering with the RSGB documentaiton - still in print.
"reviving classic designs"
GB, and any others that might have an answer:
Do you remember a good old CB antenna that was made years ago, called the "AstroPlane" I never could figure out that antenna from an electrical standpoint. It was fed at the center, and drooping down from the feedpoint was a pair of aluminum tubes that flaired out until they terminated in a circle of aluminum tubing. About the same distance above the feedpoint, another aluminum tube terminated in a capacity hat made of crossed aluminum rods.
The whole thing worked like any conventional ground-plane antenna, but the company that made it ( forgot who, a Loooong time ago! ) also had a beam version of the antenna, called, naturally enough, the "AstroBeam"
The point of all this is that the "AstroPlane" reminds me a lot of a "j-pole" antenna, with just a few mechanical differences, probably enough to add some marketing "pizzazz" and make it more attractive to the prospective buyer.
Anybody have enough insight on that antenna to tell me what exactly the engineers were trying to do with that antenna design? Would it have enough merit to be tried on 10 and scaled up for some of the other bands, as well? Or is a plain old chunk of aluminum or copper tubing just as good for a driven element for a vertically polarized antenna?
Just curious, but I betcha someone can give us a good answer!
73 from Jim AG3Y
06-27-2003, 08:39 PM
The name you're trying to think of is AVANTI, a.k.a. Avanti Research & Developement, Inc.. I don't believe they're still in business... In addition to the Astro-Plane and Astro-Beam, they also produced the PDL-II, and the Moonraker beams.. I've still got one of their catalogs and it illustrates the many antennas offered by Avanti.
I have never had the opportunity to "dissect" one of those Astro-Planes so I too, don't know what made 'em tick.. The first time I saw one, "J-Pole" came to mind...
A quick google.com search - I found your AstroPlane antenna - which is now being imported from CTE in Italy supposedly this was their former "Top-One" model (now discontinued?).
English web page: http://www.cte.it/home_eng.asp
26.000 - 28.000 mhz
CTE Italian Import all-metal Astroplane-copy base station antenna
06-27-2003, 10:19 PM
The skeleton slot, along with a true slot, antenna can be made for use on any frequency. The gain is usually about 2 to 3 dB over a dipole.
There were some amateurs actually loading up the rear window in their automobiles as a vertically polarized true slot for 2 meters. Although I never tried to do this, I understand that some amateurs were able to get a decent performance from this arrangement.
Whew, Glen, I sure would hate to think how much RF was radiating into the back of their heads! Hope they weren't running high power!
Yup, slot antennas are real good radiators. Use them for UHF Television Broadcasting EVERYWHERE !
As far as the AstroPlane, I sure hope one of you guys with your antenna analysis software packages can model one for us! It would be fascinating!
73 from Jim AG3Y
06-28-2003, 01:03 AM
That was back in the days when a "high powered" 2 meter mobile had 10 watts output! Most were running under 5 watts.
Ahh .. 10 watts !!
.... the days of the Regency HR-2B, Icom IC-22A, the Genave, Heath HW-202, #converted GE & Motorola gear (crystal) and the early synth. -- Icom IC-230, IC-22S, Heath 2036? .
30 to 40 watts was "high power". #The Heath 40 watt amplifer kit for 2 meters was very popular at the time.
My first homemade mobile antenna was the conversion of a popular Antenna Specialist CB antenna into a 5/8 wave 2 meter antenna. #Worked great !
06-28-2003, 02:32 AM
More like back in the days of the Gonset Communicator II and III, the Heath Twoer was the "latest & greatest", etc.! FM was usually a Motorola FMTU-30D transmitter and FMRU-16V receiver ("twin casket" sets!).