Topband: Delta Loop

[Tom Rauch]

Hi All,

There certainly isn't any free lunch with antennas, and we always 
have limitations or tradeoffs on what we can do compared to ideal 
situations.

The problem with comparisons is we often compare antennas using 
very poor test methods or protocol, poor test situations (like 
comparing antennas placed too close together), or compare 
against poor reference antennas. 

What we actually observe, even if we do it carefully, is just how 
much better one less-than-perfect antenna works compared to 
another less-than-perfect and unknown performance antenna.   

> For some reason the loop does not reradiate noise as much as grounded
> verticals do.

That can only be two things. It can be from low efficiency, since 
efficiency directly affects the "capture area" and the amount of 
signals (signals behave no differently than noise) re-radiated. It can 
also be that the delta was by luck or by design placed where the 
receiving antenna was in a "null".    
 
> >Seems to me that the vertically polarized delta loop would benefit
> >greatly from a conducting ground screen underneath each of the
> >vertical sections 
> 
> That's true as long as the ground screen extends many wavelengths away
> from the antenna, where the reinforcing reflection occurs. If the
> screen is right below the antenna it might reflect some of the
> radiation straight up.  

It is also true because the electric and magnetic induction fields 
below the lower wire of a delta is very strong. The electric field 
actually is most concentrated at the voltage maxima, and so it 
would be important to have a ground screen all along the lower 
wire, not just at the "vertical" ends. 

Often overlooked is the fact loss problems are due to concentration 
of fields in the earth. There does not have to be a "path" to the 
earth with a wire or connection for there to be significant induced 
ground loss. 

The voltage on the lower wire of the delta would have a strong 
electric field impinging on earth below the wire, as well as some 
magnetic field.

Walk below one sometime with a fluorescent tube while 
transmitting with modest power, and you will see what I mean. It is 
exactly the same problem as occurs with a few elevated radials. 

The horizontal low wire of the Delta is acting
> rather like a pair of end-to-end radials I think, and when George
> Brown invented the groundplane antenna it had just two ground plane
> elements, and he added two more only because the customer thought they
> must be necessary.

Even a groundplane with four radials has significant current on the 
shield of the coax because the groundplane is inadequate, and 
even a four radial wire groundplane has pattern distortion caused by 
radiation from the radials. You can clearly see this on modelling 
programs, and I've measured significant coax shield current on four- 
radial groundplanes. I always add a choke balun on four-radial 
groundplanes, because the terminal voltage of the groundplane 
common connection is never zero, it is always significant, causing 
current to flow on the coax shield.
 
> >If it were up to me, I would consider putting some radials underneath
> >the delta loop to improve efficiency.
> 
> But the efficiency of a dipole in free space is close to 100%, and
> this probably applies to a wire loop. I think you have to be careful
> about the relative definitions of efficiency and effectiveness.

In free space the efficiency is near 100%, not near earth. .05wl 
from very good earth a dipole is less than 20% efficient.
 
> The point Bill made that you can open the apex to separate the
> vertical elements is worth remembering. This may also help keep the
> apex away from a conducting support, which is essential. But don't
> open the current-reversing point of the baseline and leave the
> baseline wires "in line". I think the ideal "squash" is .4 base line,
> .3 each slanting wire.

It would depend greatly on antenna base height above earth, and 
ground characteristics. The larger the baseline, the more gain from 
increased directivity. The larger the baseline, the more ground 
losses from increased current in earth below the horizontal wire. 
With either a perfect conductor for earth or a perfect dielectric for 
earth, loss would be minimum. Someplace between all of this in 
the real world is an optimum, but you can be sure that optimum is 
far less than what a monopole over a perfect ground screen would 
provide.   

I'd wager the vertical delta is not much different in efficiency than a 
two or four wire elevated radial vertical at the same base height.
73, Tom W8JI
W8JI@contesting.com