Roomcap -THEORY Remarks 2                                                                              
14th.July 2014 : UPD: 18th July 2014                                                                               
Felix Meyer HB9ABX                                                                                    

Remarks to SWR / Bandwidth / wrong formulas

The bandwidth of an antenna is an important property.
The bandwidth may say a lot about an antenna, but this may be understood
wrongly.
These formulas show the relation of: B = bandwith und Q = quality factor,
F0 = resonant frequency and SWR. 
   
 
               
   
An antenna can be looked as serial resonance circuit, and the
Rs above become the in the antenna Rs as Rv + Rs.
  
  
The bandwidth B of an antenna can be measured by reading the SWR.
The above indicate formula for B is right only, when measuring at SWR = 2.62 ,
when at SWR = 1 at resonance, and at frequences F1 and F2 at SWR 2.62 .

Many authors of antenna articles indicate SWR = 2 or SWR = 3, in order to
calculate the efficience, the loss-resistance and radiation-resistance.

This calculation is wrong, as they did not consider, that the formulas are only
correct, if the SWR = 2.62 (e.g. in
"Low Band DXing").
Study the text from VE2AZX, where you find the explantion in
VE2AZX-Q-factor.pdf .

The bandwidth B says does not, if an antenna is good or bad.
We have do know the values Rv and Rs, then you say if good or bad.
You have the relation Rs/Rv, then you knows what need.
The bandwidth increase if:
- Rv high cause a broad bandwidth = bad antenna
- Rs high cause a broad bandwidth = good antenna
therefore to need: Rv small and Rs large, then we have a good antenna!

(This is applies for serial equivalent circuit)

If you understand this one, then becomes to be an expert. 




Supplement 18th July 2014
Once again: Bandwidth 
Below the graphics from "Low Band DXing, ON4UN".
There is explained, that from 40 degree length up a usable antenna results, as the
bandwidth becomes so small, and the efficiently so low, that a small antenna will
not serve to you ...
Over 13 pages of calculation and computer produced results, and long tables to
design a large antenna.
 
Thereby the impact create, that only with such large antenna constructions will
you obtain good results.
In the graphic above you see, that below 15 degrees height there is no chance
to obtain usable antenna with shorter antennas. ...
But in the ROOMCAP we are in the left area of this graphic!
With the measures "current over planes" and "counter planes" we obtain a
high relation Rs/Rv, and therefore the efficiency of over 85% , and
a large bandwidth, and a low angle radiation.
To this see the remarks 1 .

Here you see the bandwidth of short antenna: Roomcap 40m
   
 

Import:

- no current to ground, as there the power is lost!
- no wrong "adaption/matching" where the power is lost!
- no antenna who produces "near field", as this field does not radiate!
- no "buried radials", as the current goes to ground!
 

 
                                                            

Back