UPDATE 09 August 17: A DX Engineering hi-Z preamp was added at the vertical and remotely powered via bias-TEEs at both ends of the coax run. It appears to work well after a bit of testing. We shall see! (The coax replaces the twisted pair feedline and matching transformers. The preamp is high-Z in and 50 ohms out. Preamp requires 12 VDC.)
UPDATE 13 December: I added 3, 50′ radials spaced 120 deg. and another ground rod to the system described below – RX time needed to see if anything improved… ————————————- all good, see 18 Dec 15 blog entry.
The latest chatter within the 630m group is regarding so-called ‘High-Z Receive Verticals’ – so using pretty much what was available here, I have built one. Time will tell how well it works, compared to the loops and E-Field probes I already employ at WG2XKA…anyway, here it is…
Steve, VE7SL published a design on his site, based upon early work by Dallas Langford – ‘Dallas Pages’. Essentially, a vertical antenna is grounded thru the primary side of a step-down transformer with a turns ratio of 10:1. Thus, the impedance ratio of this transformer is 100. The secondary impedance is multiplied by 100 in this case, and reflected back to the primary, which is what the vertical ‘sees’. The received signal is directed back to the shack using a balanced pair of twisted conductors; thus greatly reducing noise pickup and common-mode problems. The shack end of the twisted pair terminates at the primary of a second transformer with a 1:1 turns ratio. If the secondary of that transformer is connected to a receiver or preamp with a 50 ohm input impedance, the vertical sees at least 5K ohms, hence the ‘High-Z’ vertical. Follows is the link to Steves site, with more information:
I happened to have about 30′ of free standing telescoping aluminum tubing that was originally set up as a TX vertical – the bottom piece was previously insulated by PVC tubing. I decided to use this.
The vertical itself is clamped to a base consisitng of a four foot piece of black iron steam pipe, with three feet driven into my rocky ‘soil’. It took four attempts to find a spot where it could be driven in three feet – very ugly!
After cleaning the tapered sections, the vertical itself was erected and secured with muffler clamps, then aligned to be truly vertical.
Any old toroid may not work. The core material must be appropriate for low frequency operation, such as Mix #75, 77, or J. The J mix is probably the most desirable, rated down to 1 kHz, followed by #77 mix. Cores scrounged from any old computer switching PS may be suitable. I had #77 cores of appropriate size on hand, which I used while waiting for J mix cores to arrive. The vertical transformer consists of 80 turns of #26 wire as the primary, with 8 turns of #22 wire as the secondary, wound on the cold (grounded) end of the primary. The 80 turn primary is wired in series with the vertical and ground. I used three interconnected 8′ ground rods, again having to find nearby spots that would accept them! The 8 turn secondary is connected to the twisted pair leading to the shack, about 40′ distant. NOTE: The twisted pair must not be grounded at any point!!
The shack-side transformer consists of 8 turns bifilar wound on another, smaller Mix #77 core. One wind connects to the twisted pair (ungrounded!), and the second wind drives the receiver or preamp, which may be unbalanced.
The vertical transformer was assembled into a Bud box along with the far end of the CAT-6 cable. Again, the secondary, balanced transmission line must float between the vertical and shack transformers!
IN THE SHACK
Eager to try this, the shack end of the twisted pair was temporarily tacked to the 8 turn, bifilar wound, output transformer. The unbalanced secondary drives a low Z, low noise preamp – designed by Larry W7IUV / WH2XGP.
Unfortunately, an impending front coupled with standard daytime noise did not make for the best test sceneraio. The RX vertical appears to perform well at first glance, NDBs received on my loop and E-Probe were of equal or better strength with similiar daytime noise. WWVB at 60 kHz was booming in at S-9 above S-4 noise…time will tell, but the effort expended seems worthwhile right now. I’ll update as testing proceeds.
UPDATE 01 Jan 2016 — The RX vertical has turned out to be a good performer. Noise pickup is less than that of the E-Field probe, and about the same as the 12′ untuned loop, which has a figure-8 pattern. I have decided to stay with the 80:8 transformer on Mix #77 material, as described. This is the antenna used when the NYE European opening struck! See Blog.
PREAMP MAY NOT BE NECESSARY
I also tested the vertical without a preamp – coupling the unbalanced side of the shack transformer directly to the 50 ohm RX input. Signals and noise were down somewhat, obviously, but my initial observation is that a preamp isn’t absolutely necessary…