> Just to clarify . > In an antennae, the radiation is produced by the interaction of the > magnetic field (current in the conductor) and the electic field (voltage to > ground). Since the surrounding medium is > air, which has an rel permittivitty and rel. permeability close to 1, the > velocity ratio is also close to 1. I humbly submit that you *must* be right about this. I was under the impression that the inductance of the antenna radiator itself would result in a decreased velocity factor. Obviously, I was quite mistaken about this. ;-) > If a "top hat" is added to the antennae, the capacitance is increased (due > to the increase in area) and the current in the conductor increases, at the > expense of the peak voltage. This also reduces the resonant frequency of > the antennae and allows a shorter annennae to be used at lower frequencies. Whilst this is true, I think that the primary reason "top hats" are used is to lower Q and increase usable bandwidth (by reducing SWR). > Similarly, a loading coil at the base of an antennae will increase the > inductance and similarly decrease the resonant frequency. This is most definitely true. > Now, if a matching network is applied to the feed, this may also add > capacitance and/or inductance to the circuit, also reducing the resonant > frequency and resulting in a slightly > smaller overall structure - possibly the 15% or so you mention in an > earlier email. Capacitance would serve to electrically shorten the antenna, resulting in an apparant increase of the resonant frequency. Inductance, as you stated, reduces the resonant frequency. > Also , a 1/4 wave antenae on an "infinite" ground plane has an impedance of > ~ 37.5 ohms but on a more normal ground plane, in practice, the impedance > is slightly higher. Because it is not infinite however, the L & C are > slightly unbalanced resulting in a reactive component. Therefore a > magnetude match is the best acheivable (without additional components) and > this requires the length of the antennae to be reduced slightly. Shortening it would add capacitive reactance (I do believe) resulting in an "apparant" feed point impedance (Z + Xc) that is closer to 50 ohms. This is one of those antenna areas where something appears to be a benifit (lower SWR), but in all reality it is detriment (anntenna is not being used at resonant frequency (by resonant frequency I mean Xc = Xl = 0) This would result in lobe distortion in the radiation pattern, heat generation in the antenna, and other inefficiencies, I believe. > At least , that is how I understand it. Sounds like you have a pretty good handle on things. ;-) The one point I would like to make, for the benefit of others reading this, is that antenna matching circuits (tuners, as most call them) do not make an antenna work "better", they just make the transmitter happy. An antenna works best when operated at it's resonant frequency, regardless of the feed point impedance that results. Having made a "few" antennas (for transmitting as well as recieving), it has been my experience that as frequency goes up, the bigger the gap gets between textbook formulas and real world working values. HF antennas are easy to work with, VHF gets to be a pain very quickly. ;-) > I don't even want to consider thinking about how this impacts radiation > patterns and gains etc. Oops, I kinda brought that up already. ;-) > > Richard P > > > > > Er. > > The quote refers to a transmission line - not an antennae. > > The correct length for an antennae is very close to the calculated free > > air value -but may be changed by loading coils, "top hats" and matching > > networks etc. > > If you use a transmission line e.g. coaxial cable as part of the matching > > network then you do need to take its velocity factor into account for > this > > . > > This velocity factor is a result of the capacitive and inductive > properties > > of the materials (pretty much entirely the diaelectric coefficient of the > > insulator) in the transmission line. Solid polythene gives a factor of > > about 0,67, expanded polythene a ration of up to about 0.84. Since an > > antennae is insulated by air, the velocity factor is close to 1.00. > > > > Richard P > > Hmm, I respectfully disagree. The antenna itself is a continuation of the > transmission line. While it may not have as low a velocity factor as the > coax feeding it, it still has inductance (and consequently reactance) and > will manifest that as a velocity factor <1. I think the fact that top-hats > and coils have the effect that they do proves this out. > > michael brown (really sticking my neck out now) > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu