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Calculating/designing phasing sections for vertically stacked 5/8w 2m elements?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC8QVO, Sep 23, 2018.

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  1. KC8QVO

    KC8QVO Ham Member QRZ Page

    All,

    I am looking at building a stacked 5/8w vertical for 2 meters. I have seen a lot of matching stubs used in J-poles that are electrically 1/4 wave. Some are coils, some are hairpin style, and some are hairpin style wrapped around a circle as opposed to straight out, and then some are 1 turn coils. Another source I found in a review of a Diamond stacked 2 element 5/8w base antenna they determined the "phasing inductor" was about .7uH. I don't know how you can give a measurement in inductance that is going to amount to any accuracy in an RF environment. Inductance and electrical wavelength don't necessarily go hand-in-hand. Impedance is a factor of resistance and reactance. Without knowing the resistive component of the impedance the .7uH inductance is irrelevant. I suppose one could work that backwards to estimate the resistive component of the complex impedance, but there are still too many variables in everything for this to be any reliable/accurate measurement.

    I would be curious on methods to match the feedpoint also, that do not require the matching stub like a J-pole. I want to build my antenna around PVC pipe - 1" for the base/longest section, then taper to 3/4" for the top section. I don't mind wire or a coil wrapped outside of the pipe, I just want a good method to phase.

    As far as tuning the input for this - I was thinking about making a capacitor out of two parallel pieces of insulated 12g Romex strands taped together, then trimmed to tune, and a relatively fixed inductor (I think it is going to be easier to tune the capacitor than the inductor - so leave the inductor fixed in place/value) to complete the L network. Does anyone see any issues with this? I can measure both inductance and capacitance so the values can be known values. In the end, though, the SWR is going to be the telling tale. I may be able to get an antenna analyzer that shows the impdance (complex), but that is about a 50% shot at this point (if it can be found and if it works...).
     
  2. KJ4ADN

    KJ4ADN Ham Member QRZ Page

    Steve,
    are you trying to phase 2 or more 5/8wl verticals?
    Just curious what the goal is, because if it's an omni directional, vertical polarized with gain for stuff other than repeaters... there's a guy who designed a vertical dipole, and flattened the pattern to something like a pancake (I built one). Terrific gain compared to a 1/4wl and OSJ-pole antenna, but when the wind blew and it moved/tilted, the signals were "gone". If I had a link, I'd send it... it was a really neat idea.

    Problem with phasing VHF-UHF bands, is just a little bit of trimming, makes a huge frequency change... It'll test your patience! It's like tuning a high-Q antenna, with that really, really narrow "sweet spot", the higher the frequency, the smaller the adjustment. This is the realm of a multi-channel scope, IMO, if you're really trying to get it right. And it kinda needs to be done "onsite" to account for variables (at least to see what happens to the lowest vs highest elements, when other objects come into play).
    But, if this is for multiple frequencies on the 2m band ... how are you going to adjust your phasing line(s) for the frequency changes? What was in phase at one frequency, is now out-of-phase (remember the bandwidth gets even narrower on higher frequencies, compared to HF bands).

    KJ4ADN - Bill
     
  3. K7JEM

    K7JEM Ham Member QRZ Page

    I am not sure how to calculate the phasing section. Like you, I have seen it done many, many different ways. The simplest is just a coil of wire, similar to a loading coil, and that seems to work OK. About a year ago our club had to temporarily move a 2M repeater from one building to another, and use a temp antenna about 12' off the ground, on a support that would not hold our DB224. I found a Tram fiberglass antenna that had three 5/8 wave elements separated by coils. The antenna itself is pretty flimsy, but I figured it would last for a year or so, maybe longer. The antenna is around 14' long, and I really didn't expect much from it, but it actually seems to work quite well. The coils used in it are about 3-4" long, and .7" in diameter, so each one had many turns to get to the QW of line delay.

    Your capacitor idea will probably work. I have been thinking of doing something similar, using 2 or 3 of the 5/8 elements, then put in an old Stationmaster type antenna shell. I was thinking of using a short piece of coax cable as the series capacitor, I think around 4 or 5 pF would be needed. Since 50 ohm coax is around 30pF per foot, it would require a length of around 2" or so. A shunt coil of around 400nH should make it work.

    Probably to start, a coil of wire around a QW long would be a good place to begin. I don't think it is extremely critical, if you can adjust the length of each element. Some of these designs use the same coils to cover 144-174 MHz. Obviously, the phase is changing quite a bit over that range, but the specific antenna lengths get the tuning right. Sometimes the element lengths are quite a bit off from 5/8, but they work.

    Attached is a chart for the Hustler G6. They use a different coil about every 6 MHz, but they could probably stretch that to 10 if the wanted to:

    http://mcarcoh.org/ke8rv/g6-144b.pdf
     
  4. K4SAV

    K4SAV Ham Member QRZ Page

    A better description of this antenna would be a collinear. "Stacked 5/8 wave verticals" brings up visions of independent 5/8 wave verticals, one over the other, and somehow phasing them, which would be a nightmare.

    If you are going to build this, I recommend you do a NEC analysis of it before attempting a build. That will save you a lot of experimental time. The phasing between sections can be done with an inductor or it can be done with a folded piece of wire. This link will get you a long way towards determining the design.
    https://www.hamradio.me/reviews/diamond-cp22e-antenna-review.html

    Jerry, K4SAV
     
    NH7RO likes this.
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Might be easier to build a Stationmaster knock-off using fat coax (RG-213/U works, but fatter coax works better!) for the radiator and phasing. There's very little guesswork involved if you have an accurate ruler :) and the array comes out to be 50 Ohms without need for any specific matching system.

    I've built these from RG-214/U and also from RG-218/U (the 218 has less loss and results in more gain) and once tuned, pulled it through thinwall (SCH 40 I guess) PVC tubing for structural suppport, with an end cap at the top drilled for a hook so it can be hoisted up into a tree or other high support (transmission line comes out the bottom) and they work very well. Maybe a $20 total investment for a 5.5 dBd omni antenna that's 20' tall (2m version) and doesn't weigh much. No coils or capacitors needed.
     
    Last edited: Sep 23, 2018
  6. KC8QVO

    KC8QVO Ham Member QRZ Page

    Jerry - that is exactly what I am after - 5/8 wave elements, stacked vertically, then a phasing section between them to get the current stacked correctly.

    The ~ .7uH x 1.5 inch long phasing coil mentioned in that article is a very "loose" estimate. I can probably make a coil that is similar and has that .7uH value, but is that going to pay off in practice with the correct phasing at the design frequency? How would I test it? Would a hair pin match between the elements be a more sure way to get the phasing correct? Or a single turn coil?

    I have an antenna analyzer to use (shows complex impedance - resistance + reactance , not just SWR - MFJ something or other), inductance meter, capacitance meter, and SWR meter. I have a 2 channel oscilloscope also, but I don't know that it is going to be of any use for the antenna project.

    To start the build process I was thinking of starting with a single 5/8 wave element, then match the feed point with an L network using the antenna analyzer. Once I get the L network tuned to the element make the other 2 elements (3 elements total) and put them in the circuit independently. Then trim the elements to match them to the already matched L network. Once I have the 3 elements set then work on the phasing and stacking. I am not sure what to expect with that. In theory, it sounds like a good plan but with the phasing a variable anything could happen once I put a second element on and then again with the third element.

    I am assuming... (I know, never assume) that if the phasing is off the impedance of the antenna feed point will change and thus the SWR will change. So that means that (if this theory is correct) once the L network and elements are tuned I could adjust the phasing to tune for SWR.

    Is my theory with this on track?
     
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I'd say it's not on track.

    The elements could be out of phase and provide a perfect impedance match if the base network is correct.

    That's why I recommended stacked 1/2-wave elements instead, making the entire antenna from coaxial cable as discussed in this article, so there's literally no room for error or misadjustment (if you know the V.F. of the cable used -- I use only "mil-spec" cables like RG-213/U, RG-218/U, RG-17/U, etc. so the V.F. is 0.66 and guaranteed by the specification); the phase shift networks are all "coax" and if you have a ruler and know your coax, there's no way to make a mistake:

    http://www.repeater-builder.com/antenna/wa6svt.html
     
    NH7RO likes this.
  8. K4SAV

    K4SAV Ham Member QRZ Page

    You are describing making independent verticals and stacking them but that reference to the 0.7 uH choke is for a collinear, something entirely different.

    Stacking multiple verticals and phasing them to produce gain in an omnidirectional array is not going to be easy. That's the reason I said it will be a nightmare. Here are the reasons that will be a nightmare.

    How are you going to get the feedline for the top element past the elements below and keep the feedline out of the field of the antennas below? You could put the verticals on arms at some distance from the tower and place the feedlines at right angles to the verticals. Of course the tower will alter the pattern due to being close to the antennas, so don't expect an omnidirectional pattern.

    I would not recommend using a matching network for each antenna unless that matching network can be identical for all antennas. If the matching networks are not identical they will affect the phase of the current to each element and that will alter the pattern and reduce the gain. All verticals will have to be identical. All the verticals will not present the same impedance to the feedpoints (matched or not). You will have to use a current forcing feed system for each element and then add a matching network at the point the feedlines are combined.

    People don't usually build this kind of antenna and I haven't analyzed it either. You will have to do some simulation to determine the optimum stacking distance. I'm guessing it will be somewhere close to 6 ft if the antenna uses horizontal radials. Actually you can't go much less than that for 5/8 wave verticals. I'm also guessing that stacking 5/8 wave verticals won't offer any advantage to stacking 1/4 wavelength verticals. A NEC analysis will provide the answer. In that analysis I would also want to investigate if sloping radials offer any advantage to flat radials. That would only be of academic interest because I would never consider building something like this or recommending it to someone else.

    You will probably need chokes on the feedlines at each antenna to keep feedline common mode currents off the feedlines which can mess up the pattern. The radial side of the antenna will need to by isolated from the support arms holding the antennas to keep common mode current from running down the metal support. Common mode current problems with phased arrays are much more critical than for a single element antenna. You could also solve that common mode problem by using two sets of radials on each vertical. A second set spaced 1/4 wavelength below the top set. Although thinking about that again, I doubt that there will be room for that with 5/8 wave verticals unless the optimum stacking distance is compromised.

    Building a collinear is much easier because all these problems disappear.

    Jerry, K4SAV
     
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I didn't get that from what he wrote, at all.

    I assumed (?) he is making a vertical omni collinear with elements stacked one above another, attempting to feed the elements in phase; thus, the phase delay network between elements (as opposed to physical spacing between elements, which works better but takes up a lot more space).

    This is tricky because "matching" hasn't much to do with phasing. The elements could be 180 degrees out of phase and cancel each other completely and still match just fine.

    That's why I recommended the "all coax" design using 1/2-wave elements and phasing sections, a la a "Stationmaster" design. Very difficult to screw it up.
     
  10. AA7EJ

    AA7EJ Ham Member QRZ Page

    OT comments / question.
    What is the primary purpose of this "stacked vertical " (?) antenna?

    Single radiator, such as 1/2 wavelength vertically mounted "J pole" radiation pattern is omnidirectional - suitable to access local repeaters.
    2 collinear antennas of same physical makeup will have omnidirectional radiation pattern suitable to access same local repeaters.
    On UHF and higher frequencies RF propagates in "line of sight" fashion - hence the governing factor is the actual antenna height , not its radiation patter or some minuscule "gain" between single radiator and collinear radiator(s).

    In theory you will not notice measurable "improvements" of your 2 meter coverage by adding another vertically polarized radiating element.

    Personally and as an experiment I would vote for "slim Jim" collinear (made from 450 ohms ladder /window feed line) and physically mounted 180 degrees out of phase and electrically fed "in phase".
    For phasing , ladder line is easier to calculate / adjust then coax, and for mechanical reasons.

    Your opinion / mileage will of course vary.

    73 Shirley
     

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