30 meter problem on my screw driver antenna.....

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB4MNG, Jul 11, 2019 at 10:31 PM.

ad: L-HROutlet
ad: l-rl
ad: MessiPaoloni-1
ad: L-MFJ
ad: Left-2
ad: Left-3
ad: Subscribe
  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    Had some time to play around with my tarheel ground mounted antenna. So far, its great on 75, 40, and 20. I experimented with an added coil and have made a few contacts on 160. 30 is a problem. I can utilize my antenna analyzer make slight adjustment as I work toward 3o meters and it just looses its ability to lower the swr down on that band. Other bands no problem. I've played with a shunt coil with and added radials with no success.
    Tarheel advises it as an impedance issue and suggested a very extensive radial system to get it on 30.

    Do you know of anything else to try, will just live with the issue that put down the required radial system Tarheel suggested.

    thanks kb4mng
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    What kind of problem is it?

    No SWR dip at all anywhere in the 10.1 MHz band? Or does it dip someplace, but just not much?
     
  3. KB4MNG

    KB4MNG Ham Member QRZ Page

    It will slightly dip from max out, meter pegged.
     
  4. WA2ZQX

    WA2ZQX Ham Member QRZ Page

    I use the M-100 high-power model and was able to lower my SWR to 1:1 on all bands except 20-meters 1.1:1...after speaking with Robert at Tarheel. My ground installation has the antenna mounted on a small tripod to clear an HVAC unit. Initially high SWR issues on three bands and thought I probable ruined the shunt coil by the time I called Robert, Hi! He suggested that I add at least one counterpoise for each band that was a problem...That did the trick for me. FYI, I only have room for a 180-degree ground radial-field, but the antenna works like a champ now. My tunematic-lite controller helps hitting the sweat spots. Of note, I use an LDG-1000 Pro2 antenna tuner and run anywhere from 50-700 Watts. Soil is sandy with high winds off the Gulf of Mexico, so I weight the tripod with rocks in sand bags and also guy the mount mast. I presently use 26-ground radials and 6-counterpoises, I believe two counterpoises on 20-meters.
     
  5. NH7RO

    NH7RO Ham Member QRZ Page

    If you got your antenna to sweat you're hitting it way too hard! Go easier! Antennas have feelings, too.
     
    N4MU and KC8VWM like this.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Does your antenna analyzer display complex impedance? If so, what is the antenna feed point impedance at the point where you see the lowest SWR on 30m?

    Shunt inductors can work great IF the real portion of complex impedance is less than 50 ohms and the reactive portion is slightly capacitive (e.g. -j10 ohms to -j30 ohms or so) which basically means that at resonance the antenna input impedance is real (definition of resonance) and less than 50 ohms. If the impedance at resonance is greater than 50 ohms then a shunt inductor won't help but other matching methods could.

    So at the point where you see minimum SWR on 30 meters, what's the actual feed point impedance in the form of R +/- jX? That measurement should be made right at the feed point or as close as possible or use something like the OSL method to account for any transmission line between the feed point and the actual measurement point.

    Also, what's the final configuration of the Tarheel antenna when you see this problem? I know you were playing with adding longer whips and capacitive top hats for operation on lower frequency bands. So how do you have this configured in terms of extra whip or cap hats when you see this problem on 30m and what's the complex impedance on 20m at best match in this configuration?
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Following on the post above:

    Without knowing the details, I suspect a couple of things are happening:

    - The screwdriver plus extra upper mast, cap hats and possibly whip is electrically longer than 1/4 wavelength on both 30m and 20m making matching difficult but helps matching and overall antenna efficiency quite a bit on lower frequency bands.

    - I suspect the SWR measurements you're making are at the shack end of the coax feed line to the antenna so you're getting some helpful impedance transformation on 20m due to length of the coax but not on 30m. If so, this is a lot like a G5RV or ZS6BKW where the feed line transformation helps the matching on some bands but not on others. If the SWR measurements you're making are taken right at the antenna feed point then this is obviously doesn't apply but it would explain why you could get a good match on 20m but not on 30m if the root problem is an electrical radiator (all the mast, cap hat and whip) that's longer than 1/4 wavelength on those bands.

    A clue that the root problem is an electrically long upper antenna would be that you get your best 40m match with the screwdriver down near the bottom end of its adjustment range which would indicate that you're not using on a lot of inductive base loading on 40m to achieve best match. If that's the case then removing all inductive loading would still likely result in a long antenna on 20m and probably on 30m as well.

    You can test this by running the screwdriver controller so that you have minimum loading inductance (typically the shortest setting of the screwdriver assembly) and perform a wide frequency sweep with your antenna analyzer looking for the SWR dip. If the best SWR (or resonance even if SWR isn't great) occurs at a frequency lower than the 20m band or lower than the 30m band then the basic antenna elements including capacitive top loading are electrically longer than 1/4 wavelength on those bands. IOW, the frequency where you get resonance with the screwdriver coil inductance set to its minimum position is the highest frequency for 1/4 wave use for this antenna configuration.
     
  8. KB4MNG

    KB4MNG Ham Member QRZ Page

    My equipment does not show impedance. All my radials are buried so I dont think adding the correct length radial will help but will give it a try. I've tried the 102 in whip, 6 ft whip, and one much shorter with the same result.

    thanks
     
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Did you try the shunt matching coil on all of those whips or just the longest whip? And was it just the whip above the screwdriver coil or was there a mast, cap hat and then the upper whip?

    If it was just the screwdriver and the 6' whip with no extra cap hat loading or upper mast then a shunt coil should help the match but it does have to be about the right amount of shunt loading, too much inductance in the shunt coil and nothing will change, too little and the feed point impedance will become inductive and the match will generally get worse.

    Even without the ability to read impedance I'd try a broadband SWR sweep with the screwdriver coil in it's minimum position (least loading inductance) and measuring at the feed point. If the SWR dips at a frequency below the band you want to operate on, then the upper antenna elements are too long for that band. A screwdriver antenna is like a very basic matching network, all it can do is add inductance which electrically lengthens the antenna. If the antenna is already too long electrically then the screwdriver coil cannot help.

    Maybe your problem is different but the common problem with higher frequency matching on a screwdriver or other base loaded mobile antenna is an upper element that is too long electrically, especially if you get a decent match on lower frequency bands.
     
    NH7RO likes this.
  10. KB4MNG

    KB4MNG Ham Member QRZ Page

    Ill give that a try with the shunt coil, thanks
     

Share This Page