New aerial for Winter Hill.
On 31/01/2018 18:20, R. Mark Clayton wrote:
On Tuesday, 30 January 2018 23:51:15 UTC, wrote:
On 30/01/2018 20:24, Brian Gaff wrote:
Actually many logs had a slightly rising gain with frequency to compensate
for losses due to the higher frequencies.
I don't know how that could be achieved Brian. The gain of a log is set
by the tau factor, which can't vary along the boom.
I think it might depend on which end the cable is connected. Usually it is the large low frequency end, but some have the connection at the short high frequency end.
No, the cable has to be connected at the HF end. That's fundamental to
the operation of the aerial. What you've seen is examples where there's
an internal feeder from one end to the other.
To explain in a bit more detail, for a given frequency only a small zone
along the boom of the log periodic is active. The zone is centred on the
dipoles that are at or near resonance at that frequency. Consider two
adjacent dipoles, somewhere about half way along the boom. The incoming
signal is in the middle of the frequency range of the aerial, so the two
dipoles are approximately resonant and are therefore in the active zone.
Feeder reversal and the distance between the two elements gives a phase
shift of 360° between them. In other words, for a signal coming from the
‘front’ of the aerial (only) the signal on the two dipoles is additive
if it is collected from the front of the boom-cum-transmission line.
This is why log periodics are directional, and why the feed-point is at
the sharp end. It’s possible for three or even four dipoles to work
together in this way. A smaller ratio between the lengths of adjacent
dipoles means more dipoles are near resonance for a particular
frequency, which is why the gain is higher for such an aerial. There are
two things that prevent the dipoles that are shorter and longer than the
resonant ones from playing much of a part in reception. One is that they
aren’t resonant and the other is that the spacing between them does not
produce the 360° of phase shift that is essential for the aerial to have
gain and directivity. The spacing between the directors on a yagi can be
varied widely (and often is to help feeder matching) but with a log
periodic the spacing has to be much more closely defined, and has to
take into account the velocity factor of the twin booms.