Matching a Vertical with an UNUN
Matching a Vertical with an UNUN avatar

When adding the radials to my 160 T-vertical (72 ft vertical + 56 ft top loading t-wire on each side for a total electrical length of 128 ft) I kept notes on the SWR as I added 3 – 4 radials.  As predicted I had a very good direct feed match starting with only 4 radials…e.g. between 1814 – 1824 the VSWR = 1.1: 1.  But the reactive part of the load X = 8.  According to the books we have not reached anywhere near  peak efficiency.  As I added groups of radials the numbers appeared to get progressively worse…but I believed the experts that the efficiency was getting better.  But with 10 radials X = 0 over much of the band and the SWR was generally 1.2:1. Skipping all the in-between measurements, when I reached my goal of 32 radials (I could only expect 0.3 dB gain going to 120 radials the nirvana)…the VSWR was now  a flat 1.7 – 1.8 between 1800 – 1900…but X = 0 over much of that range….so the load impedence was almost all resistive (R).  On the MFJ 259B R = 38 – 26 over the same range.  But the VSWR being somewhat higher than I would like, I researched putting a 1.56:1  UNUN (unbalanced antenna to unbalanced coax) right at the antenna base.  This would match 32 olms impedenace of the antenna (almost all R ) to 52 ohms of the coax.  Balun Designs, LLC makes baluns of all types and  can  handle  up to 2 KW.  At $69 I thought I would try this approach for the first time.  It took a couple weeks to get it.  Sent USPS Priority.  Looked nice and installed it today.  VSWR is now 1:1 on 1820 and > 1.5: 1 from 1800 – 1870.  At 1900 it’s 1.8:1.  The MFJ will also measure Percentage of Transmitted Power (a backhanded way of stating VSWR) and it peaks at 99% and drops only to 90% at 1900 kHz. I worked ZL8X this morning BEFORE installing the UNUN.  I suspect that this mod will make little difference if any in DXing.  But it would appear practically  all the watts are now going into the antenna.  I think this is probably a good thing.   — W1XX

3 comments on “Matching a Vertical with an UNUN
Matching a Vertical with an UNUN avatar

  1. SRI, Ed. I wish I had photos…but my camera went beserk some time ago and my phone doesn’t take pix. But I can report that adding the UNUN has not ruined my signal….improved it if anything. This evening on 160 worked HK0GU, 9L7NS, ZD8O, and TO3A…thru big pileups and mostly on the 1st or 2nd call. So things are cooking very nicely. I can recommend this approach to getting a good match and pouring all the watts into the antenna. 73!! — John, W1XX

  2. John,
    Thanks for the update. It sounds like you have a winner. I agree with everything you said. My only question would be the MFJ box. I’m not sure how it works, and what it’s telling you. But then I’m used to using a box that costs several thousands of dollars more than the MFJ box. I’d be interested in bringing out a high-priced VNA and taking some measurements both with and without the UNUN.
    I’d also like more info on the source for the UNUN. – Mike, K1DM

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