[Antennas] Q loaded versus unloaded

Wed, 1 Oct 2003 20:50:23 -0500

Jim, the problem is that for a resonant circuit, the formula you quoted uses
the bandwidth at which the output of the circuit is down 3 dB.
Usually the "bandwidth" of an antenna is expressed in terms of its SWR range
such as 2:1 or below.
I agree that a "high Q" antenna such as a small loop has a narrow bandwidth
because of the low losses required for any efficiency at all.
One way to measure the "Q" of an antenna would be to measure the field
strength at the resonant frequency and then to find the frequencies at which
the field strength has fallen 3 dB.
The problem is that this "Q" probably varies with where you are in the
pattern when you take the measurement.
Another approach might be to find the ratio of the radiation resistance to
the loss resistance, if possible.
So, I end up wondering what you would do with the Q value if you had it?
Bob probably has some ideas on this . . .
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"Starting the 58th year and it just keeps getting better!"
[email protected]
73/72, George
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"Starting the 58th year and it just keeps getting better!"
[email protected]
----- Original Message ----- 
From: "James Duffer" <[email protected]>
To: <[email protected]>
Sent: Wednesday, October 01, 2003 5:23 PM
Subject: Re: [Antennas] Q loaded versus unloaded
> For resonance isn't there a formula that goes something like Q =Fr/Fbw Q
is
> equal to the frequency of resonance divided by bandwidth. In other words
> the narrow bandwidth is result of high Q.
>> Now since this applies to tuned circuits, could it also be applied to
> resonant antennas????? If so, then you can easily measure the Q of your
> resonant antenna.
>> 73, Jim, wd4air
>>> >From: "Robert Lay (W9DMK)" <[email protected]>
> >To: "Hue Miller" <[email protected]>, <[email protected]>
> >Subject: Re: [Antennas] Q loaded versus unloaded
> >Date: Wed, 1 Oct 2003 09:10:50 -0400
> >
> >
> >Dear Hue,
> >
> >
> > > Has this been already discussed here?
> > > What kind of figures for Q would you typically get for
> > > a wire antenna without any loading? For example,
> > > a one-wavelength antenna would act like a high-z
> > > tuned parallel resonant circuit. Assuming wire size
> > > is large enuff so ohmic resistance isn't a substantial
> > > factor, what might be a typical ballpark figure Q?
> >
> >It's a factor of so many variables that it is not possible to give a
> >quantitative answer. Here are some considerations that must be taken into
> >account:
> > Since the reactive components of the antenna impedance (seen at the
> >feedpoint) are dependent upon the electrical length of the antenna, the
> >inductance and capacitance components will vary.
> > Since the ohmic portion of the antenna impedance (seen at the
> >feedpoint)
> >is a function of ground losses and the ohmic losses in the antenna and
the
> >electrical length and the height above ground, the "loading" will vary.
> >
> >Everything considered, it would be necessary to use antenna modeling
> >software in order to obtain a quantitative value for a given set of
antenna
> >parameters. All of that having been said, the bottom line is that the
> >higher
> >the antenna, the lower the ground losses, and the larger the wire size,
> >the
> >lower the copper losses. Both of which will contribute to a higher Q. The
> >bottom line is that for a half wave, resonant system in free space there
is
> >a nominal value of Q that will obtain, and there is very little that you
> >can
> >do to improve or degrade that significantly with traditional antenna
> >construction practices.
> >
> > > Would proximity to ground affect this "Q"? If so, which
> > > way? In other words, would a dipole at lower wavelength
> > > above ground have narrower or broader bandwidth?
> >
> >I think the answers to those questions are indicated above.
> >
> > > Also, forgive a dumb question, but where can i find
> > > the formula to calculate impedance along a wire, in
> > > one of the ARRL or other books?
> > > Okay to just point me to a good but not extremely
> > > mathematical book. Probably not an EE text.
> >
> >The best answer that I can give is to suggest two things - one, there is
no
> >easy formula available that I know of, but EZNEC software is the accepted
> >answer to all those questions. The other is that you are probably worried
> >about factors that are not easily controlled. The more important factors
> >are
> >the radiation pattern of the antenna and the question of getting your
> >transmitter's available power to be transferred to the antenna
efficiently
> >and to then be radiated efficiently. Using traditional antenna
construction
> >practices is hard to beat - ie., get it up as high as is possible and in
> >the
> >clear, as much as is possible and practical. All of the factors that work
> >to
> >the contrary of those guidelines will usually reduce the efficiency of
the
> >system.
> >
> >Bob Lay (W9DMK) in Dahlgren, VA
> >http://www.qsl.net/w9dmk