430 MHz DIY Yagi problem. Ideas needed.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by BX2ABT, Dec 2, 2018.

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  1. BX2ABT

    BX2ABT Ham Member QRZ Page

    I used the VK5DJ's yagi calculator software to calculate a DL6WU 7 element yagi for 436.5 MHz. Folded dipole, one reflector, 5 directors, all 8mm alu-tube, fully insulated on a 1-1/4" alu-boom. BALUN made according to the software's calculations with RG-58A/U, with the pigtail some 10 inched long.

    I use a Kenwood TH-F7E HT with a Daiwa VHF/UHF SWR meter for testing. The dipole is resonant at @436 MHz (SWR 1:1), so I assume dimensions are good. Now, when mounting it on the boom with the other elements the following happens.
    • SWR meter directly connected to the pigtail: SWR 1:1.7 on 436 MHz
    • SWR meter connected with 3 meters RG-58: SWR 1:1 on 425 and 443 MHz, SWR 1:3.0 on 436 MHz
    • SWR meter connected with 2 meters RG-58: SWR 1:1 on 427 MHz and 451 MHz, 1:3.0 on 436 MHz
    Two resonant frequencies when using a coax line! The transmission line and its length seem to have a profound effect on the resonant frequency(ies). What is going on here?

    I know above HF things get a little more finicky and I am new to VHF/ UHF, so please could you give me some advice on what mistakes to avoid and how to properly test and later mount the antenna so that the whole system is resonant?

    BTW, from my tower to the shack I use 75 feet of RG-213 as transmission line.

    Cheers and 73 de Hans
    BX2ABT
     
  2. KC8VWM

    KC8VWM Ham Member QRZ Page

    You have to copy the design "exactly" or else there will be some variation exhibited in your own result.

    This specifically includes the same exact method used to secure the elements mounted on the boom.

    What's going on here is SWR readings in itself are not an indicator of antenna efficiency or performance. In your case I suspect your transmission line is radiating RF and needs an effective choke installed at the feed point. After all, transmission lines are terrible antennas so try to keep the RF radiating from the antenna elements instead. The choke you choose for this purpose needs to be effective for use on UHF frequencies.

    Also I suspect part of the problem is because RG-213 transmission line exhibits an attenuation loss of 5.2 dB at 450 MHz / 100ft. This means you are losing a lot of your transmitter output power in your transmission line and I think there are far more efficient transmission line choices to use on UHF frequencies.
     
    Last edited: Dec 2, 2018
  3. BX2ABT

    BX2ABT Ham Member QRZ Page

    For testing I use a couple of meters of RG-58, as stated in my original post, not RG-213. I know about line loss, but here in Taiwan we don't have the luxury of multiple outlets selling various types of coax. RG-213 is most common and relatively cheap. For reception I use pre-amps at the antenna anyway. I did follow the design exactly as it rolled out of the calculator +/- a millimeter, so there should be no problems there. I've just posted a photo here if you want to check it out.

    So what is an effective choke for 70cm antennas? You can't just slap on a ferrite choke and in the literature I haven't seen much mention of chokes for UHF.

    Cheers,

    Hans
    BX2ABT
     
  4. KM3F

    KM3F Ham Member QRZ Page

    The match at the antenna terminal is not flat 50 ohms or close enough.
    Therefore the coax length becomes an influence on the reading at the end of the coax.
    One meter difference in coax is the 'whole' world at 432 because the change is a whole wave length or part of.
    This length change converts impedance every quarter wave length so it is highly dependent on where the SWR meter is in the length of the coax..
    This also causes common mode current on the coax outer shield.
    You can see this by applying TX power, watch the SWR meter while you move your hand along the coax.
    If you see a change it's because your hand alters the common mode current the SWR meter detects.
    .
    The only way to get close to a match is to use an antenna analyzer that has the ability to operate on the 432 band.
    Connect within an inch or so of the antenna and check the match allowing for hand effects.
    Once the match is close to 50 ohms +/- a few, the coax length should be less critical except for the normal losses per length.
    I built a German design likely from the same source, several years ago and it's a flat match on a COMET CA500 MKII analyzer.
    I have used this meter to test power splitters/combiners and a whole host of VHF/UHF equipment and antennas with good results.
    My experience tells me I don't like any match types but Dipole feeds.
    Oher may not agree but that's the way it comes out and works for me.
    Good luck.
     
  5. BX2ABT

    BX2ABT Ham Member QRZ Page

    Thanks KM3F. I had to digest a little what you wrote. So my antenna is not 50 ohms at its designed frequency is what you are saying. Or could it be the mis-match between the antenna and the balun? In other words, the resonant frequency of the folded dipole and the balun doesn't match, thus resulting in the coax becoming part of the radiating section instead of being a transmission line only.
    The only experiment I could do today was to hook the antenna up with 76 feet of RG-213 and connect it to my shack set (IC-820H). I measured the following:
    So between SWR high and low there was a difference of 1/3 lambda. Strangely SWR didn't get above 1:2 this time, but it still isn't right.

    I don't have an antenna analyzer and buying one is above my budget. So how did people measure/resolve these kind of problems in the days before all this fancy measuring equipment?
     
  6. BX2ABT

    BX2ABT Ham Member QRZ Page

    In the above post the image didn't get through. Will try again later.
     
  7. BX2ABT

    BX2ABT Ham Member QRZ Page

    Thanks KM3F. I had to digest a little what you wrote. So my antenna is not 50 ohms at its designed frequency is what you are saying. Or could it be the mis-match between the antenna and the balun? In other words, the resonant frequency of the folded dipole and the balun doesn't match, thus resulting in the coax becoming part of the radiating section instead of being a transmission line only.
    The only experiment I could do today was to hook the antenna up with 76 feet of RG-213 and connect it to my shack set (IC-820H). I measured the following:
    [​IMG]

    So between SWR high and low there was a difference of 1/3 lambda. Strangely SWR didn't get above 1:2 this time, but it still isn't right.

    I don't have an antenna analyzer and buying one is above my budget. So how did people measure/resolve these kind of problems in the days before all this fancy measuring equipment?
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    BX2ABT, if you're using 76 feet of RG-213/U as a transmission line connected to your antenna at 430 MHz, the SWR measured at the transmitter would never exceed 3:1 no matter what the load was.

    You have about 7.6 dB return loss in the cable. If you disconnect the antenna completely, SWR should measure about 2.5:1 or so.

    Of course, the same thing would also happen with an antenna analyzer used at the end of 76 feet of RG-213/U at 430 MHz, so the "instrument" doesn't matter.

    Based on what you posted above, it seems your antenna is pretty well tuned for the 430-440 MHz region; for "improved performance" overall, I'd use a less lossy transmission line. 76' of RG-213/U plus two connectors (I'll say 0.1 dB loss per connector as a guess) should have about 3.8 dB loss in one direction -- pretty wasteful.
     
  9. KM3F

    KM3F Ham Member QRZ Page

    Checking the antenna should be done on a short coax at a multiple of 1/2 wave length of the operating frequency.
    Trying to measure on a long length of lossy coax covers up the true antenna feed impedance.
    The power applied is greatly attenuated traveling to the antenna.
    The reflected power experiences the (same losses) coming back to the SWR meter.
    The results are the losses greatly mask the true antenna impedance.
    Put another way, the reflected power coming back is greatly reduced making the SWR appear lower than it might be at the antenna.
    This can fool you if your not a where of this action.
    Also said another way, if you look at a long enough coax length that has enough loss at the frequency and the end is either open or shorted, the SWR might be very good without an antenna even connected.
    In this case a wide sweep with an analyzer would appear quite flat and good that is not usually the case with most resonant beam antennas.
    Reason is the load presented by a long coax has more R resistance loss than X reactance and would be most of the reason for a flatter SWR reading.
    A long run at VHF/UHF frequencies should use the larger Heliax type cables to the antenna.
    If the beam is on a rotor, connect around the rotor with LMR 600 or other low loss flexible coax to maintain low losses.
    You would gain by the difference in loss per 100 feet between the 213 and the lower loss Feed line.
    That can be a significant number of DB over a long length.
    This gain, by using the lower loss feed line, is both ways in Tx power left to feed the beam and the radio Rx signal from the antenna.
    As frequency goes up, feed lines get more specific and often more expense if you want the better performance.
    Good luck.
     
  10. BX2ABT

    BX2ABT Ham Member QRZ Page

    I took your advice and made a straight pigtail half a lambda long and a new 4:1 balun with a one lambda long pigtail. My thinking was that a folded dipole should have an impedance of around 288 ohm, which would give an SWR of around 1:6 if no balun was applied. I then measured the SWR and forward and return power over the 70 cm band and a little beyond. The results....

    [​IMG]
    A flat SWR over a 14 MHz range, but forward and return loss varying widely. Am I right that at around 441 MHz the antenna seems to perform most efficiently?

    I then used the newly made 4:1 balun and re-did the test.

    [​IMG]

    Still a rather flat SWR and again it seems 441 MHz is where the antenna performs most efficiently. Am I right that I made the dipole is simply too short for my intended frequency of 436.5 MHz? And that as a result the balun and directors/reflectors are wreaking havoc?

    BTW, in both tests I put my hand on the pigtail while transmitting and I couldn't see any influence on the SWR or forw/refl power.

    I'm curious how you interpret these results and what you think has to be done next. I hope you can spare me your thoughts again.
    73 de Hans
    BX2ABT
     

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