Sunday, November 16, 2014

Low Band Receiving Loops and the Evolution of the Trapped Inverted L

The chill in the air tells me winter is on the way.  With winter comes low band fun!!!  This winter I am sporting a new Antenna Dinamica (AD - Terminated loop receiving antenna.

I ordered the antenna last winter but only recently got to put it up.  I installed a short pole in the back yard and mounted a Hygain CD45II Rotator on top of it. On top of the rotator, I installed 10 ft pipe (1 1/2" aluminum electrical conduit) and the AD loop.  Remember, this loop is a directional antenna so it needs to be pointed at the transmitting station.  Because it is terminated, it is a broadband antenna so it works great on 160, 80, 60 and 40 meters.

My main low band transmitting antenna is a trapped inverted L.  I made the trap from RG400 coax and PVC pipe.  The trap is tuned to the low end of the 80 meter band.  The antenna sits on a DX Engineering radial plate ( and 20 radials varying in length  from 20 to 125 ft.  The impedance of this antenna works out to being about 17 to 18 ohms so I use a 2:1 impedance transformer to get this up to a respectable 35 ohms or so.  I tune this antenna with a Palstar HF-Auto ( antenna tuner.  The antenna works quite well as a transmitting antenna but can be quite noisy for RX.  So I needed something else, hence the AD loop.

Since the AD loop presents a loss instead of gain, I have installed a DX Engineering preamplifier ( inline at the receiver.  I feed the antenna with RG-6 coax that has negligible loss on 160 and 80 meters.  Therefore, having the preamplifier at the RX instead of the antenna is not a real issue.  The beauty of the DX Engineering preamp is the noise figure, at 3.5dB it is fairly decent. The low coax loss does little to ruin that noise figure. Initial tests of the antenna involved tuning in local AM broadcast stations and rotating the CD45II.  First thing I noticed was I could effectively shut off a station by pointing the antenna away from it.  WOW!!  On the air tests on 80 meters was rewarding as well.  C21GC was on the air, couldn't even hear it above the high noise at my location on the inverted L, the AD loop pulled the signal out of the noise and got me a fine new country on 80 meter CW.

So back to the inverted L....  I have really spent a lot of time with this antenna.  I initially started with only a 130 ft wire and no transformer. When attempting to tune the antenna with the HF-Auto on 160 things were great with 100 watts.  When attempting to tune the antenna on 80 meters the Palstar was really unhappy.  I used a RF Experts AA54 antenna analyzer ( to find an alarmingly high impedance on 80...  So what I learned, 1/2 wave long wires are inherently high impedance beasts and probably not the best antennas to attempt to tune with an auto tuner.  Even worse, when I tired to apply high power (1Kw) to the antenna on 160, the tuner SMOKED!!!  So what else I learned,  17 ohm impedances are tough on even the best of tuners at high power levels.  I started thinking of what was required to get the 160 meter impedance up and the 80 meter impedance down.

Enter the 80 meter trap.

I initially bought a pair of Unadilla ( 80 meter traps.  I did some simulations of the antenna in EZNEC and determined the appropriate spot to install the trap.  So the newly configured wire was hoisted into the air and it was time to test.  The data from the AA54 was quite promising.  The resonance points closely matched my EZNEC simulations and the impedance looked good on 80 meters now ( it was still pretty low impedance on 160).  I tuned the antenna with the Palstar and all was good.  I tuned the amp into the dummy load and then applied 1KW of 80 meter power to the antenna.  Ok not bad, life was good things were happy.  So now its time to try a contact...  I found a station and sent my call,  no answer, tried again, no answer but this time I got an SWR alarm from my LP100 watt meter and the HF-Auto takes off on a tune cycle.  OK what has happened.  I pulled the coax off the back of the Palstar and attached it to the AA54 antenna analyzer.  I ran a 160 meter sweep and saw no problems.  I ran an 80 meter sweep and noticed the SWR was extremely high.  I ran a wider sweep and saw that my resonance dip had moved from roughly 3.55 MHz to some where around 6 MHz.  NOW WHAT!!!

 I waited for the light of the next day and pulled down the inverted L wire to find a large crack in the side of my trap.  I replaced the trap and hoisted the antenna back in the air just hoping I had a bad trap.  On a lark, I disconnected the trap inductor and measure the capacitor with an ohm meter to find it measured 11 ohms!!!  Back to EZNEC, I inserted an 11 ohm resistor where the trap was and re-ran my simulation and matched exactly the results I saw with the antenna analyzer.

So with the antenna back in its original functional state, I made a second attempt at power application.  Long story short, the trap burnt again - almost instantly. I contacted Unadilla about power handling prediction for the trap only to be told that they were not appropriate for the application.  Seems that these traps were designed years ago when output powers were lower and few hams owned amplifiers.  For most applications, these traps are fine... just not mine.

Enter the coaxial trap....  I used a piece of RG400 and 2 3/8" PVC pipe to build the trap.  The design data was calculated using a piece of software called "Coax Trap" written by VE6YP (  The PVC pipe was capped off using pipe caps with eyebolts fastened to the centers.  Stainless steel hardware was used through out.  The trap was tuned to the low end of 80 meters using an MFJ259 with the grid dip coils (MFJ-66) by adjusting
the lay of the coils on the PVC form.   The trap was waterproofed using self sealing rubber tape.  The trap was installed in place of the now broken Unadilla trap and the antenna was hoisted into place.  The new antenna was subjected to repeated applications of 1Kw on 80 meters and has survived.

Still we have an issue with 160M.  I want to run higher power into the antenna on 160 with out blowing up the tuner.  So I ordered a balun designs ( 25 to 50 ohm unun.  I installed the transformer and all is now fine up to 500 watts.  Not sure if I want to attempt any more power and possibly damage my tuner again.  I may later just replace the auto tuner with a manual one that will be more forgiving of my mistakes.

So why so much power.  Inverted L's are great antennas but are hopelessly inefficient due to the low radiation resistance without a very extensive ground system.  I estimate that I am burning roughly 1/2 to 2/3's of my power heating my ground system.  I already have about 2000 feet of wire in the ground system and my XYL is not amenable to me copper plating our yard.  So sadly I am dealing with the effficiency problem by burning more power.  All in the pusuit of a good country count on 160 from a residential lot.

Until next time