[600MRG] For discussion: Re: Top loading a vertical on LF/MF

[Eric NO3M]

Based on actual data from modeling on my recent new vertical:

67 ft vertical w/o top loading: 0.3896 ohms radiation resistance

67 ft vertical w/ 8 x 30 ft top loading, sloping approx. 45 deg.: 0.753 ohms

Assuming a ground loss of 30 ohms, you end up with 1.28% and 2.44% 
efficiency respectively.  So with top loading, you gain roughly 3 dB, 
including reducing some coil loss in the base loading.  If your ground 
losses are higher, less of a difference.  I have approx. 9-12 ohms 
ground loss on 630M, so the difference is even better than above.

The best way to figure out the right top loading is to model it. 
Anything longer or shorter than 30 ft on my 67 ft monopole based on the 
wire slope would reduce radiation resistance.

Now, here's a dose of reality.  The power limits are based on ERP/EIRP.  
So if you can push more power from the transmitter, a more efficient 
antenna is moot.  Use as much power as needed to reach your limit.  
That's also why directional arrays in an ERP/EIRP limited situation are 
unnecessary, increase ERP concentrated in a certain direction, now you 
have to pull back power to be "legal".

73 Eric

On 11/27/2013 06:18 PM, Neil Klagge wrote:
>   To the 600m gang:
> The reason for initiating this discussion is that, for me and perhaps
> others, it is not feasible or possible to add top loading wires to a 630m
> vertical without sloping the load wires downward a considerable amount.
>
> I know that when an antenna is “folded back on itself” there is
> cancellation of some of the radiation from the antenna. This is certainly
> the case for top loading wires that reach toward the ground down to less
> than half of the height of the vertical section.
>
> The question I find myself asking is this: “Is what I will gain in ERP
> worth the effort it takes to add sloping top wires because of the
> cancellation that will occur.” I wonder if 1 or 2 dB gain is worth it at
> the receiving end.
>
> I would be interested in feedback from those who have any experience with
> both situations: Namely, top loaded verticals and “topless” verticals  ;-)
>
> 73, Neil Klagge, WG2XSV, W0YSE
>
>