Balun or choke effect on resonant frequency

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4ZDI, Nov 17, 2009.

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  1. W4ZDI

    W4ZDI Ham Member QRZ Page

    On two separate antennas, one a dipole with 4:1 balun to ladderline, and on a ground plane vertical an 8 turn airwound coax on a 4in. pvc, I found that I couldn't get accurate readings on the MFJ analyzer.

    In the case of the dipole, no matter how much I trimmed the antenna ends, it always indicated that I needed to trim more. On the vertical, the resonant frequency shifted 200KHz when I added the choke.

    What's causing this, and how do I correct (or get around) the problem?

    Thanks,
    Pierce W4ZDI
     
  2. KA5S

    KA5S Ham Member QRZ Page

    Open wire line may be used with a dipole, but it is not matched to the dipole, and its length results in further impedance transformation. If you want to tune the dipole with an antenna bridge on the ground, you'll need a half-wave length transmission line (electrical length, not physical) OR a matched coax with or without a 1:1 Balun at the antenna. A 4:1 Balun is often used (mismatched) at the lower end of open-wire line with a tuner nearby.

    The coil should not change your vertical's resonant frequency, but if it does, it indicates a substantial part of the ground current is flowing down the shield. When you add an effective choke, it can no longer do so. Add some radials at the vertical and this effect will be reduced.

    Cortland
    KA5S
     
  3. W4ZDI

    W4ZDI Ham Member QRZ Page


    Thanks Courtland. I was using a 4:1 balun between the antenna and the 400 ohm ladder line, assuming that would be the correct match. I was thinking about reducing the off-resonanant losses by using ladder line to the tuner instead of the coax. So, I gather I should use a 1:1 balun with 50 ohm coax at the antenna connection for tuning purposes, then when set, change to a 4:1 and ladder line? Makes sense. That was a W2DU 4:1 balun, BTW.

    Now for the ground plane vertical. (Long story. Change the page if already bored)
    This is the antenna that shouldn't be. I have a good ground plane 20 meter vertical out in the woods at the end of 125 feet of 213 coax. Wanting something higher and closer, I chose the top of the roof over my ham shack, well knowing there would probably be an RFI problem if used on my Flex 5K,
    but would probably be OK for the FT-450 on PSK 31.

    I started with 12 ft of half inch 6061 tubing with five feet of a slightly smaller size 6 feet long spliced into it for a total of 17 feet antenna, knowing that I would have to trim probably six inches off from the bottom. I started with four radials 16.5 feet long drooping to their ends about two feet lower than the base of the antenna, whose base was about three feet above the roof peak and six inches above the metal tripod mounted on the roof crest. PVC pipe supported the antenna and insulated it from the tripod.

    The first measurement and calculations indicated I needed to add three feet to bring it down from about 16MHz to 14.2 (approximate figures). NO Way. I tried shortening radials, moving radials, eliminating radials - no luck. I removed the antenna tubing and opened the splice connection, cleaned and re-riveted it with four or five rivets. No change, still three feet too short.

    The next morning I said, OK, have it your way. I added three feet to the bottom of the tubing. Result: Brought it right down into the 20M band! I removed two 16.5 foot radials and improved it more, leaving only two at 180 degrees apart. I slid the tubing down a bit at a time until I had the following figures:


    Freq SWR R X

    14.000 1.3:1 37 0
    14.050 1.2 39 0
    14.100 1.1 41 0
    14.150 1.1 44 0
    14.200 1.0 46 0
    14.250 1.1 48 6
    14.300 1.2 50 10
    14.350 1.3 52 13

    This is the 19.5 foot vertical for 20 Meters that I tried to put the choke on. I've used it for several days now with at least a 3dBm gain over the antenna in the woods and no RFI problems with PSK. The only thing unusual about the structure is that about 8-10 inches of the tubing below the coax connection runs down through the four inch support hole in the top of the tripod. There is probably 1/4 inch gap between antenna tubing and the 1 inch metal tripod ring which would certainly add capacitive coupling, but then again, there is only about an inch of antenna in close proximity to the ring.

    I'm quite happy with the end result, but I"m aware that something is certainly very wrong. Can you tell me what could cause such a large error, yet give such good end results?

    Thanks for any inputs,

    Pierce
     
  4. KA5S

    KA5S Ham Member QRZ Page

    Hi, Pierce.

    In the case of the dipole, if you are already using open wire line and a tuner, just cut it to formula and run ladder line to the tuner. If you really want to resonate it, you can use a half wavelength(a) or full wavelength(b) of coax (electrical length, which for RG-213 would be about (a)22 feet or (b)44 feet) to the MFJ. But if the tuner will compensate for any slight errors, why go to the trouble?


    You spent a lot of time on the roof. Ny guess is that nearby wiring under the roof, and perhaps metal gutters, of just the wrong length, detuned your radials, making them look like a capacitance (and thus requiring additional radiator length to resonate). Remember, radials need to be well clear of anything conductive and have their ends insulated to work in an elevated position. You could keep adding radials until wires underneath don't have as much RF on them.

    It's not unusual to have some of the vertical element inside a supporting insulator. And it's IMO usual to have less than 10 inches, with coax connected near the bottom (you can drive a screw into a hole drilled through the PVC into the aluminum.) Add the transmission line coil; it'll help keep RF out of the shack. Then if it works for you, retune as needed and keep making contacts!

    Have fun!

    Cortland
    KA5S
     
  5. W4ZDI

    W4ZDI Ham Member QRZ Page

    Cortland, again many thanks for your help.
    My reason for wanting to resonate the dipole was 1) it is an inverted V and 2) I wanted to bypass the tuner. I've read that an inverted V would be perhaps 5% shorter than normal so I wanted to compensate for that. I'll take your suggestions and do the tuning with 50 ohm coax.
    OK on the explanation for the extra three feet on the vertical. There isn't any wiring inside the 12X12 roof interior, but there is a 12 inch heating duct (steel coil circular expanding type wrapped in foil) that comes in under the roof from the house right under where I put one of the radials. Duh! Live and learn.. But the antenna stay as it is - works too good.
    Pierce
     
  6. K9ZMD

    K9ZMD Premium Subscriber QRZ Page

    Did you have the MFJ analyzer set up properly to test balanced feed line? If not, your readings will be off. You need a balun between the analyzer and the balanced feed line. You also need to use only internal batteries to power the analyzer to take measurements on balanced line. Afterthought: yes, you probably did those things correctly because they are mentioned in the instruction manual. So why were the readings screwy then?
    Maybe that could account for the screwy readings. You do not need a balun between balanced line and the feed point of an inverted vee antenna. First, because balanced line to a balanced antenna is fine just as it is; no correction needed. Second, because you simply do not care about the impedance mismatch at the feed point. Sure that's going to cause a high SWR on the balanced transmission line, but so what? The reason you use balanced line is because its loss is so low that it can be disregarded. Resonance is also irrelevant with this type of antenna; it will radiate just fine whether resonant or not. Yes, there will always be an impedance mismatch at the feed point, but your transmatch will deal with that mismatch at the shack end of the antenna system, presenting your transmitter with an acceptable impedance.
    Consider bypassing your transmatch only when you have a coax-fed resonant antenna. Being able to operate with the tuner bypassed is not a reasonable expectation if you are using balanced feed line. Whether or not your antenna is resonant, whether or not you use a balun at the feed point, and whether or not you use a balun at the shack end, you will not be able to bypass your transmatch and find a good match for your transmitter on most frequencies with a balanced transmission line. Even if you transmit at exactly the resonant frequency of your antenna, the impedance is unpredictable (depending a lot on transmission line length) and a good match for your transmitter may be more by accident than by design.

    Yes. As you gaze at your MFJ antenna analyzer, are you thinking that a low SWR reading is an indication of resonance? If so, that can account for a lot of things (including partial responsibility for screwy readings for your inverted vee). Low SWR does not equal resonance. The vertical length that you've found to give a low SWR is not resonance; instead, it is the length that gives a feed point impedance close to your coax impedance. Check the instruction manual again for using that analyzer to find resonance.

    Regarding removing radials to improve the antenna: First, you don't tune a ground plane antenna by lengthening, shortening, or removing radials. Second, if you remove radials, you are increasing the current flow (and attendant IR loss) in the remaining radials. Your SWR may improve (due to increased IR loss), but that doesn't mean that your antenna performance has improved. Low SWR does not equal antenna effectiveness. With just two radials, you actually increase the potential for RFI in your shack due to higher RF currents in those two radials, and greater flow outside the braid of your coax. Adding radials divides the return current, lowering the amount flowing in each radial (and diminishing current flow outside that coax braid). This reduces the total IR loss and also reduces radiation field strength from the radials (spreading it over a wider area). Instead of worrying about the radial length and nearby objects, use all 4 of them at 16.5' and concentrate on using your analyzer to correctly identify the best vertical length for resonance.

    If you want to actually tune those radials at some point in the future, you can also do that with the analyzer; however, don't try that until you are really down with finding resonance. The formula length is good enough for the time being. Tuning radials with an analyzer is also the subject of totally new thread. 73

    Gary, K9ZMD/6
     
  7. W4ZDI

    W4ZDI Ham Member QRZ Page

    Thanks for you inputs Gary. I've learned something with this forum. Yes, I knew that Reactance at or near zero was the object in tuning the antenna and the chart shows I got pretty close - even if approached from the wrong end.

    Pierce
     
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