Feb 18, 2008

The Backyard 160m RX Reference Vertical

Ever since I moved to my new qth (dec '05) I have been trying to get the best possible 160m reception on the available backyard space. But with 7 x 8 mtr there's ample room for ....heuh.. just about anything.

So let's just try making the best of it, right?

We do have some years of experience in building & using different kinds of RX antennas. Especially during contest, like K9AY, KAZ, Low dipole, single and phased staggered beverages.
Some at contest sites, some on a roof top at a 60mtrs AGL, others in fieldday style.
The more different antennas we tried, the more we learned. But at some point it became apparent that the 'next new antenna experience' is of little use if you cannot compare it to anything with 'known' specs.

Clearly a reference antenna was needed.

At my buddy's qth, where we build and try out most of our new backyard antennas, the same demand was rising. So we decided to both start building the same reference antenna.

Wanted criteria:
- small, non obtrusive appearance
- Omni directional with some low angle dx capabilities
- reliable and constant electrical behavior

After examining the several home made rx 4 square antenna setups of ON4UN, W8JI, etc. we decided to start building a single vertical.
It's doughnut shaped radiation pattern delivers relevant low angle reception next to suppressing high-angle incoming signals. Based on that it already has a DMF spec of about 4dB (refer to Low-Band DXing by ON4UN, chapter 7.1.8). Think about that for a while.

Building the vertical part
This consists of 6mtr black polyurethane coated copper clad steel wire (0.9mm dia.), taped to a 6mtr glasfiber pole. At some point we collected 6 pcs of these cheap fishing poles near the end of a fishing season in a large store. Currently they are all in use and we're gonna need more of them.

Ground connection
We hammer-drilled a 2mtr, 22mm dia. copper pipe into the ground. From earlier experiences we already knew that there are small ‘seasonal’ changes in the earth resistance. Not alarming though, but since this is going to be a reference antenna we added 4 radials. These consist of 4mtr blank copper wire, 1.5mm dia. which are buried at 10cm depth in the ground and connected to the copper pipe. Measurements during the year on a network analyzer showed that this ground setup remained constant.

Bringing the 6mtr (20 ft) rod to resonance on 1830 KHz
After several experiments with various coils on different types of toroids, we found that about 38 windings on a FT114-61 ferrite in series with the vertical wire works well. Resonance is pretty sharp though; one extra turn will shift it some 20 KHz. Impedance showed near 50 Ohms, but that's just stupid luck, otherwise we would have added a series loading resistor or a suitable x-former.

What else?
The waterproof box also holds a 1:1 transformer which consists of 2 by 2 turns of generic insulated hookup wire on a type #73 binocular ferrite. The coax is wound several times on a FT150-J (1500mu) toroid to minimize unwanted common mode noise. Burying the coax in the ground helps too. At the receiver input a similar common mode coax filter is used.

How does it play?
Actually VERY well. From october '07 onwards I am able to hear the big gun USA stations, and several world wide dx like BY, 3W, UA0, CX, VK and JA. It's not that you can tune into every clusterspot, but it catches much more than expected !
It also receives very well in the upper part of MW BCB, which is a nice bonus as it delivers several small domestic pirates above 1600 KHz.
The signal output is quite high; I have the RF gain backed down to 70's and sometimes another 6-12dB ATT on the Orion.
The slim black appearance is well accepted by both my neighbors.

Future plans?
  • building a second one for setting up a phased vertical system at 1/8 distance
  • compare with other antenna systems as planned

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