Monday, April 13, 2020

SWR problem on bike vertical

I did a test this morning of what I'd hoped would be an operational set-up for park operations accessed via bicycle and ended up regretting selling both my Elecraft T1 and Emtech ZM-2 tuners.

The set-up was this:
  • MFJ-1979 telescoping mast (17ft tall, extended)
  • Vise-grip antenna mount
  • 2 (and then 4) 1/4-wave radials
The plan was to operate on 20m CW where a 1/4-wave vertical is a few inches short of 17 feet.

With two radials laid out on the ground, I attached the mcHF to the antenna and heard a good number of strong stations. So far, so good.

Upon answering a CQ'er the mcHF's SWR meter showed 3:1 so I immediately stopped to confirm what I was seeing with the antenna analyzer, thinking I might need to decrease the length of the mast just a bit.

Resonant but high SWR
I managed to get the SWR down to 2.2:1 and the analyzer indicated that the antenna was resonant at that point, ie almost no reactance. That's great but I still need a lower SWR risk harming the transceiver.

Two more radials were then added with very little effect. A 2 MHz sweep centered on 14.060 MHz showed that 2:1 was as good as I was going to get without some sort of Z-matching network.

Moral of the story: Even a resonant antenna can have an unacceptably high SWR.

An antenna tuner would have easily taken care of this problem and one is on the way for the mcHF,
As good as it gets & not good enough
currently in the Cvirus-delayed mail somewhere between here and Ukraine.

My next bike trip with radio will include a PAR end-fed, nylon cord and a weight - hopefully this Saturday.
.
.
Posted by John AE5X at

9 comments:

  1. David Ryeburn VE7EZM and AF7BZApril 13, 2020 at 6:53 PM

    You need to get the resistive component higher than 22.5 ohms. First try lengthening the antenna a bit and put a capacitor between antenna base and ground. The resistive component will rise a tiny bit, but more important the inductive reactance will rise above 0.4 ohms and that plus the capacitor will make an L network raising the impedance somewhat. Adjust the capacitor for best SWR. You can make the capacitor out of a coax stub, far end open. If you get SWR down to 1.0 before you run out of mast length, good. If it improves but not enough even with mast at full length then instead try shortening the mast and putting an inductor between antenna base and ground. One way to make such an inductor would be with a coax stub, far end shorted. Let us know how it works.

    ReplyDelete
    Replies
    1. John AE5XApril 13, 2020 at 7:03 PM

      Excellent, David - thank you for those suggestions. I just happen to know where I can borrow a BNC-T that will make the trial& error easier. I will try lengthening first but only have 4-6 inches of mast left before it's fully extended. If that doesn't do it I'll try the stub/inductor next.

      Tnx again & 73,
      John

      Delete
    2. David Ryeburn VE7EZM and AF7BZApril 14, 2020 at 2:25 PM

      One way to lengthen it even more after it's extended all the way is to put some sort of capacity hat at the top of the mast. Try that first before going the shortening/inductor route.

      My guess is that 4 to 6 inches more of mast won't be enough to get enough inductive reactance. You're going to need about +j25, and then about -j44 in the capacitive stub. This is assuming that the 22 ohms or so resistive component doesn't change much.

      Delete
  2. Don, ND6TApril 14, 2020 at 10:07 AM

    Since it is resonant it might be good to just try an autotransformer on a small toroid. Simple, small, cheap.
    73,
    Don

    ReplyDelete
  3. Don, ND6TApril 14, 2020 at 11:21 AM

    If I may elaborate on my previous suggestion of an autotransformer:
    An efficient implementation would be a "fractional UNUN", a trifilar winding of 7 or 8 turns on a type 43 (or similarly high permeability ferrite). The size of the core would be dependent upon the power level. The "one of three" configuration should provide a nearly perfect match for stepping 22 ohms up to 50 ohms.
    I've used a similar configuration for years to great success. Very efficient and reliable. 73, Don

    ReplyDelete
    Replies
    1. John AE5XApril 14, 2020 at 4:20 PM

      Hi Don - I have seen what you describe being used to Z match stages in circuits and would be interested to trying it here but I want to use the antenna on both 17 and 20 meters. Would an autotransformer be broadbanded enough to accommodate both? I tinkered with SimSmith a bit today and think I'll just make an LC match for now. It was a fun learning process and I may try you idea on another project.

      Thanks and 73,
      John

      Delete
    2. Don, ND6TApril 14, 2020 at 4:43 PM

      Hi John,

      Indeed! That is yet another advantage. This is a very wideband solution. Great for a trapped vertical or for a screwdriver type. No tinkering required. 73, Don

      Delete
  4. Dave NewApril 14, 2020 at 1:08 PM

    I guess you found out that the resonant impedance of a vertical is typically much lower than 50 ohms. I've used a small matching coil to ground at the base of my mobile HF vertical to help deal with this. It's a 'cheat' because it doesn't affect the real radiation resistance of the actual antenna, but it makes the transmitter happy.

    ReplyDelete
    Replies
    1. John AE5XApril 14, 2020 at 4:10 PM

      Hi Dave - I was thinking it would be around 35 ohms, giving me a 1.4:1 SWR. But now that I think about it, that's for verticals whose radials taper at a 45-degree angle from the radiating element. I was surprised that adding two additional radials did very little in terms of changing the R+j values.

      73 - John

      Delete

Subscribe to: Post Comments (Atom)