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Thread: VHF Station Master Internal Dimensions

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    Default VHF Station Master Internal Dimensions

    Does anyone have the detailed drawings of the old Station Master VHF antennas? There was a ham repeater local to me that had one that resonated around 153 MHz, so I built them a 2-element folded dipole array out of parts that I had and replaced the old Station Master. The club gave me the old Station Master and I tore into it today to see what made it tick. The dimensions on the elements don't make sense to me. There is a 32" tube at the top with six 29.375" copper tubes in the middle with the matching network at the bottom that has two 19" sections. All of the elements are made out of 7/8" O.D. copper tubing. I've tried to come up with the math using a Vp between 80-95%, but it just seems off to me. This antenna is 21 feet, 5 inches long. It had moss growing out of it when I took it down. I know I can replace the original guts with new coax and a new formula, but I'm interested in the original design.

    This antenna was very old. It used slotted screws on the outer sleeve, a soldered on tip on the end and a PL259 at the base.


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    Can't help you with the math, but can say that antennas are wizard's work.
    Bow wow wow yippie yo yippie yay

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    To me it looks like they used 80% Vp and it's cut for 153 using six 1/2 wave elements and a matching network at the bottom. The top confuses me as it doesn't seem right. I can't tell how long the center conductor is or if it is shorted in the middle of the last shield or not, but the top shield is 32 inches with the cap.

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    Can you post some well detailed photo's? The last Station Master that I seen was about 40 years ago and had been struck by lightning and blown to bits. I do a lot of antenna modeling and it would be interesting to duplicate the design in a NEC model if your could post the photos.

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    Here are a few pictures. Sorry, it's in the grass. I don't have a table that's 22 feet long...lol

    It also appears that the top element shield is 32" with the end-cap, but the center conductor is only 16" as it stops right in the middle. I don't think it is shorted at that point; it just appears to be an open. There is a small hole drilled right where that center conductor ends.
    Attached Images Attached Images

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    OK. Thank you for posting the photos. It has been a long time since I have seen the inside of a station master. So yes this is a coaxial collinear array and the top section is just an open half-wave (electrical) element. If I recall the center conductor of the adjacent (next lower) section should be soldered to the outside tube (shield) of the top section and vise versa. These antennas require more elements in collinear to produce the same gain of a typical folded dipole array where all elements are driven equally and in phase due to the tapering of the RF current as it traverses each array element with less and less current the further you get from the feed point.

    There is some information posted over on repeater-builder.com in the antenna section that may be helpful, especially if you intend to restore/rebuild this antenna. Pay attention to the info about how to fix the PIM and RF Noise problem due to dissimilar metals in the matching and input section. Oh, the lower part of the antenna should also form an inverted bazooka that when combined with the radials help to electrically (RF) the array from the mounting pipe.

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    Thanks. Yes, I've read about brazing the ring to the bottom to cut down on noise and I'll do that, but am curious as to what velocity factor was used for their calculations. Doing some reverse math, I came up with 80% on the elements and 88% for the top section. 95%, which is what a lot of people claim, did not match the frequency at all. I would like to see the original build/engineering drawings instead of what I'm finding on the internet about using coax as none of those designs match what I'm seeing inside this thing.

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    You could unsolder one of the six 1/2 wave sections from the array then short one end and us a VNA or VHF capable antenna analyzer to measure the impedance at the
    opposite end. A shorted 1/2 wave section will reflect a very low impedance at the end opposite the short. Sweeping the VNA/Analyzer up and down in frequency from 153 MHz
    until you find the frequency where you measure the lowest impedance will indicate the actual resonant frequency. Now use that value to determine the length of a 1/2 wave length in free space then divide that into the actual measured length to compute the VP factor. It should be somewhere around 79-80 percent.

    It would be interesting to measure the top section. In this case I am assuming it represents an open line section so you should expect to measure a high impedance on your
    VNA/Analyzer at the resonant frequency. Same rules apply to determine the VP factor if it's turns out to be different from the lower six sections. Don't forger to calibrate your instrument to account for the line connecting it to the element under test.

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    I swept the antenna before I replaced it and it was resonate at 153 and was about 6 MHz wide.

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    The only real mystery is the top section. But I believe that the dimensions of the lower six sections equated to a VP of 79.x % VP factor that is consistent with what your measured and with the publish center frequency of 153 MHz. I'd still like to know how the top section looks if you're up to it.

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    Rain is in the forecast, so I put it back together and put it away until I get a better understanding. Here is my math for the 32" long 1/2 wave top section:

    5616 / 153 = 36.706

    36.706 * 88% = 32.3

    I tried to get a picture of the inside of the top section, but it either was out of focus or the flash did not illuminate the inside of the tube, so you can't see anything. I'm assuming that the center conductor is open (not shorted to the tube) where it ends in the middle of the tube. That center conductor is ~16 inches in length.

    I suppose I could cut new elements at 30.75" and cut a new top section at 33.875" and see what that does. It does add 10 inches to the length if I keep the same spacing between elements, so I'd have to extend the fiberglass at the top. It's going to be hard sourcing the 7/8" copper tubes. 1/2" is more readily available here. I just hate to buy a bunch and use the wrong velocity factor, especially if it's different than the original design.

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    It's my understanding that the top section often consists of two components.. The first is a shorted electrical 1/4 plus a full 1/4 radiator. Now I am not sure about the VP factor for this section but will go with 76% based upon the six 1/4 wave sections. Top section coaxial length = (((983.57/153)/4) x .76) x 12 = 14.65 inches. Now the full 1/4 wave
    radiator portions would be ((983.57/153)/4) x 12 = 19.28 inches. Adding these two values gives us 33.93 that is a bit longer than the 32 inch length you quoted in your original post. Did this dimension include the taper (pointed) cap that extends out of the top of the fiberglass radome? If it does then my assumption for the value of the VP factor is incorrect. But this gets you in the ballpark and hopefully fills in the blank about the top section.

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    Yes, the length I got included the brass cap, which added about an inch.

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    It's not the velocity factor of the element coax that reduces the length of the sections in this design (the VF of the semi air spaced sections is more like 90% unlike the ham versions made from solid dielectric coax) but the capicitive shortenting caused by the proximity of the next element.
    It is a fine thing to be honest, but it is also very important to be right

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    That makes perfect sense. So if one were to unsolder a section from the array for analysis with a VNA then you'd likely measure a different resonant frequency than
    with the fully assembled array due to the capacitive end effects of the adjacent elements. Thanks for pointing that out Astro Spectra.