All, Just as an FYI, Inductance in an antenna system makes the radiating elements electrically longer, which in turn lowers the resonant frequency. Top Hats are used to add capacitance to effectively bring the antenna system back into resonanc e at the desired frequency. This is a little oversimplified, but it is also the gist of the matter. Regards, Jim KA9QHR >> 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? 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