Receive antenna - another update

Thanks to Carlos, N4IS, for his continued support, by email and on the telephone.

Today I replaced the coax feeders with CAT5 twisted pair. I ran the wire inside the boom and the mast. This should eliminate most if not all chance for common mode noise pickup on the feed line to the preamplifier, as it’s insulated, and because twisted pair is designed to minimize noise pickup.

Using Polar plot, I was able to collect this table that clearly shows that the antenna is working mostly as it should. I did my best to rotate the antenna by hand at a constant rate. For the test I used a 500W AM station about 2 miles away. There are some small lobes to the rear that is probably interaction with something nearby.

n2nl_rx

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Posted under Uncategorized

This post was written by admin on September 30, 2009

Receive antenna update

I spent all weekend messing around with my “Waller Flag” receive antenna.

Some background: I have a remote antenna switch located on the dock right at the shoreline. All coax and control cabling are shielded, buried, and extensively grounded at the antenna switch, both with a ground rod and several radials laid into the salt water itself. From this point I feed all my antennas, both with feed lines and control cables (20m array and 80/160m switching). This setup has served me well, protecting my station and home electronics from damage when the antennas took two direct lightning hits this summer.

For my receive antenna, I ran a new length of coax from the pre-amp at the base of the mast to the ground point where I tapped into an unused coax for the 25ft run to the station. The cable is submerged in salt water for the entire length to try to eliminate common mode currents on the feed line jacket. To supply 12v to the amp, I used a piece of flooded RG-6 coax, with + on the center conductor and - on the braid. I had it laying around, so I used it. This cable was also submerged from the antenna switch box (where I tapped into 12v) to the amp itself.

All weekend long I struggled with the antenna. It did not seem to be working properly. With one AM station I would see 20db+ F/S and F/B but on another I’d see little or no difference. At night, the receive antenna seemed to hear worse than my transmit vertical. I was becoming frustrated.

On a whim, I replaced the 12v power cable with a fully jacketed two conductor cable. I grounded the jacket at three points along it’s length, by connected bare copper wire and laying radials into the salt water. Presto - my daytime noise floor with the amplifier on went from S8 to S5, an 18dB decrease in noise. Last night, for the first time, I was hearing exceptionally well with the receive only antenna. On 160m, G4ZCG was even with the noise floor on my transmit antenna (S9). On the receive antenna, without the amplifier energized, he was much weaker but still perfect copy with no background noise. Later I was able to listen to his conversation with a F9 station on 160 SSB with perfect copy. This example played out through the night. Conditions were flat to Europe, however I was able to copy several on the RX antenna I could not copy at all on my transmit vertical with the QRN. The antenna also performs well on 80 and 40m. JD1BMM on Minami Torishima was solid copy on 40m this morning well after my sunrise. I could not hear him on my 1/4 wave vertical.

So far today I have added two more grounding radials to the control and feel line shields, further dropping my daytime noise floor from S5 to S4. I plan to continue grounding until I see no more improvement, at which time I will add some more ferrite beads to see if I gain anything further.

As a followup, I am seeing some excellent F/S and F/B with this antenna now that I have corrected the common mode concerns.  AM1300 in Marathon, Florida, about 45 miles away, is S9 on the RX antenna.  Turning the antenna 90 degrees away from the station drops the signal to S4 (30dB if my K3 S-meter is accurate).  For a while I thought I had a bad electrical connection - this afternoon I am seeing spikes in the noise level to S9.  It turns out that I’m hearing the thunderstorms in South Florida.  Turn the antenna away from the storms, and the static crashes mostly go away.  Amazing!

Posted under N2NL QTH, Uncategorized

This post was written by admin on September 28, 2009

New low band receive antenna

I recently finished building a “Waller Flag” receive antenna primarily for the 160m band. The design was taken directly from N4IS’s web site, at which Carlos has published detailed instructions. The design is originally from NX4D, and both he and N4IS have had great success with this antenna.

The left two transformers are 9:1 and the one on the right is 1:1.  All three are wound on 73 mix binucular cores to W8JI recommendations.

The left two transformers are 9:1 and the one on the right is 1:1. All three are wound on 73 mix binocular cores to W8JI recommendations.

The spreaders for the two loops are actually made of PVC, something I can get cheap here in Key West.  It only needs to last one season, as I transfer next summer.  This shot is looking southeast.

The spreaders for the two loops are actually made of PVC, something I can get cheap here in Key West. It only needs to last one season, as I transfer next summer. This shot is looking southeast.

Here's another shot, looking east (towards Africa).  The antenna "sees" salt water from 45 degrees (Europe) through south to west.

Here's another shot, looking east (towards Africa). The antenna "sees" salt water from 45 degrees (Europe) through south to west.

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Posted under N2NL QTH, Uncategorized

This post was written by admin on September 26, 2009