[Lowfer] efficiency of multiple antenna systems

[Stewart Nelson]

Hi Steve and all,

I don't think that getting up to 65% is so bad.  They started with
a .395 ohm effective loss resistance.  If the antennas were truly
independent, the final value would have been .194; they actually got
.248, which is 73% of the theoretical savings.

If LowFers did as well, then an array of two antennas would gain
1.97 dB; not awful.  But I believe that the LowFer case would show
more gain.

There are two major components in the "extra" .054 ohm loss resistance
at WWVB.  One is the common component in the effective ground
impedance (what we are arguing about).  The other is the increase in
loading coil Rac, because the coupling between antennas demands an
increase in inductance.  There is not enough data in the WWVB paper
to calculate this, but as a rough estimate, if the inductance is 20%
higher, and the Q is 700, then that could account for .032 ohms.

> Actually, really seriously - please give us actual examples of practical
> applications using numbers for a typical LowFer installation - and not
> where you start from a really cr*ppy antenna that no serious LowFer uses.

*All* LowFer antennas are really cr*ppy.  It's required by the rules :)
Which is why if two LowFer antennas are 100 feet apart you won't need to
increase loading coil inductance significantly.

> > Luckily, I stumbled across some professional documentation of this
> > effect.  The station happens to be our own WWVB!
> >
> Possibly relative luck :-)     Just who is Glenn Nelson :-)?

AFAIK, Glenn Nelson is no relation of mine.  But there have been Nelsons
in the broadcast business.  My father, Morris Nelson, was one of the
developers of the NTSC standard, and had 28 patents in the field of
color TV.

> BTW, I also "stumbled" across a reference to this idea in the ARRL Antenna
> Book.     There is given an example of a 1.8Mhz system consisting of two 50
> foot antennas spaced by 10 feet (now there's a coincidence, 1.8Mhz/10 foot
> spacing - 180kHz/100 foot spacing - sound familiar? - mmm...).    There it
> is stated that the ground currents are not coupled - just how this is
> possible with two 50 foot radiators spaced 10 feet apart I cannot
> understand.    Subsequent queries to the principals of this publication (I
> will not name them as I haven't ask permission for quoting them) produced a
> lot of caveats.

I agree with you that there must be some BS in that article.  Not only
would I expect significant coupling of the ground currents, but also a
loading coil inductance requirement of almost twice the value for a single
antenna.

> A summary of the responses so far from that direction is
> that it is better to make a single radiator more efficient by itself by
> providing an adequate ground-radial under it.

I don't disagree that you can improve the efficiency of a single radiator
by adding metal.  I'll even admit that if your ground resistance is really
high, and your loading coil is pretty good, you might gain almost 3 dB
just by doubling the ground coverage.  But my point was simply that you
can also do that by using multiple radiators, and if one can find a
legal way to drive them with multiple transmitters, one would see an
increase in ERP resulting from the combination of improved efficiency
and more Tx power.

73,

Stewart KK7KA