[TenTec] Capture area (was 80 meter loops)

[ac5e at comcast.net]

It might be clearer this way. You put a certain amount of RF energy into your antenna system - which will be expended in various ways. Tuner loss, perhaps; transmission line loss certainly. And if some of the latest work on EM radiation is correct you can even wind up with much more than the generally accepted 50 percent of delivered RF lost to near field radiation. 

What is left after all the losses is the RF power you have to work with. Your antenna can pretty much put that out in all directions - or more or less aim it in one direction. A directional antenna concentrates what is left after losses in some, hopefully a useful, direction. A non-directional antenna sprays energy all over the place. The concentrated "beam" is more useful for point to point communications over a long distance. The "sprayer" is more useful for regional communications. That said......

A "full wave centerfed dipole" has a very high feedpoint impedance that will take a hellaciously big bite out of the RF budget in terms of excess feedline loss. As will ANY dipole style radiator consisting of half waves on each side of the feedpoint. Yes, such antennas are a little more directional; have "gain in a given direction;" than a half wave dipole. But you lose much more than you gain on transmit to transmission line losses; and you lose much more than you gain on receive to reradiation losses. 

And the only 100 percent successful way I know around that is a high ratio impedance matching device AT THE ANTENNA TERMINALS. A matching network starting at 80:1 or so for the soils around here at a reasonable height above ground will keep the transmission line losses down, but will introduce losses of its own. 

A 3/2 wave center fed dipole has an odd number of quarter waves on each side, and has a reasonable feedpoint impedance, as witness the success of the many who use their 40 M dipoles on 15.

On the other had a 3/2 wave END fed monopole has the same drawback as a 1/2 or 2/2 wave end fed monopole. The feedpoint impedance is outta sight! 

73  Pete Allen  AC5E


> jsb at digistar.com wrote:
> 
> > So is a halfwavelength dipole a preferable antenna than a endfed halfwave
> > for a given band?  How does an endfed fullwave or 3/2 wave antenna compare
> > to 1/2 wave endfed vs dipole?
> 
> 
> It is necessary and sufficient to compare these and other antennas
> by comparing their [more or less] realistically
> computed radiation patterns, as related to the job at hand.
> 
> It's a budgeting problem:  there is a fixed amount of
> average gain [same for any antenna] and any increase in favoured directions
> is paid by corresponding decrease in all the other directions.
> 
> In some cases a highly directional ("high gain") antenna may be desirable,
> but if various directions have to be worked without rotating
> the antenna, then a less directive antenna may provide stronger
> signal in non-preferred directions.
> 
> In other words, a beam can have some "gain" over dipole
> for some narrow azimuth and elevation ranges,
> but for all the other directions it's the dipole
> providing the "gain" over the beam, and this gain is
> as real and as effective as any other.
> 
> 
> 73,
> 
> Sinisa  YT1NT, VA3TTN
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