Topband: The WD8DSB mini-flag antenna

[n4is at comcast.net]

>> 1.  I am specifically talking about "small" loops.

 The antenna in question is not a "loop" it is a terminated loop, the diagram is a cardioid, it is very broadband 1-10, like 1 MHz to 10 MHz or 3 o 30 MHz with the same irradiation diagram. The behavior is comparable a two dipoles  in phase. Thermal noise is low as the resistor. 

>> 2.  In small loops, the ohmic dissipation of the conductor vastly overwhelms the tiny radiation resistance.

Yes , but loads loop is based on reflected wave and the one direction current dissipate on a 1K resistor ( gives you the front back and the other direction the current see a 9:1 Balun. Again it is not a loop the Q is very very low because it is broadband,

>> 3.  This is unlike a vertical antenna or a dipole, which can be considered non-dissipative. 

 Again it is not a loop with high current and high Q or a tunned loop, it is very broad band  the BW is over 10 MHz.

>> 4.  Therefore, if the conductor is at a physical temperature of 300K, its Johnson noise will correspond to that temperature.

The voltage is not over the wire in on the termination due the high SWR, one side is dissipated, the one you don’t use and the other is a transformer.


>> 7. It is extremely easy to get a 50 ohm NF of 0.5 dB with a BF981 at 88 MHz as specified by its data sheet.  All you have to do is transform the 50 ohm source so that it loads the FET with about 1,000 ohms.  I did this 40 years ago.  Unfortunately, at 1.8 MHz, the flicker noise of the BF981 dominates, so you can't get the low NF down there.

>> 8. I did this same experiment with the multiple paralleled BF981's over
15 years ago and was limited by flicker noise, which is unspecified, YMMV.  Maybe you had better devices than I did.

> pair of phase loops. A/B tests with NORTON preamps show not possible 
> to copy the weak signals I did copy with my 6xBF981.


Here I agree, I was luck , I was not expecting 43 db gain. I was looking for a good input filter and I used a T240-6 with litz wire for a Q over 500. The impedance of a 3T to 11T inductor and a very high Q provide a voltage gain at eh input , the transconductance of the gates doe the work and I got a 43 db gain single stage preamplifier with a very high IP3, the BW just enough for 40 Khz, but the extra gain make the preamp usable for 1.8 to 1.9 mHz.


>> Considering that the input of the preamp is deliberately connected to the antenna, it is hard to imagine what signal the shield is shielding the amplifier from.  Large BCB signals out of band coming up the feedline?  Is the shield still needed for QTHs that have no nearby BCB stations?

Not at all, the amp is very well protected below my desk, inside a galvanized steel box, inside an aluminum box and the preamp inside a tinplate box.  My HWF a phase pair or loaded loops, reject vertical noise with a 50 db null at the direction of the maximum horizontal gain. Adding a negative gain over 40 db, the rejection of any vertical signal is over 90 db, no manmade noise or BCB in below the MDS of my system. All my cables run inside metal conduit too. 
The transmit vertical is a folded UNIPOLE and I use the skirt to detune the TX antenna during RX.

You can find all information on WWROF webinar archive. 

73's
JC
N4IS