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[email protected] · October 27th 14, 12:37 AM posted to rec.radio.amateur.antenna

Lostgallifreyan wrote:
wrote in :

The rhombic was a big deal in it's day back when huge, empty areas were
readily available and better antennas had not yet been invented.

I like apples and strawberries, but I'll not go into it.

I had a very quick look at log-periodic antennas before I sleep. That looks
like a much more practical notion to me. I guess practical DIY might still be
limited to shorter wavelengths, but it looks like a neat, compact and solid
antenna design, ideally suited to anyone with some accurate tooling and a
need for directivity combined with a relatively broad bandwith reducing need
for adjustments. Would it be a contender against a tuned magnetic loop for a
beginner's experiment?

Like all things in life it is a trade off of various things.

The accuracy requirement, at HF anyway, is not that bad and there are
LOTS of plans for DIY log periodic antennas out there.

Upside: Basically frequency independant (over a range), all metal
construction, can directly match 50 Ohms, and gain can be increased
by increasing the number of elements and making it longer.

Downside: A high gain antenna can be quite large, require a lot of
expensive aluminum, be quite heavy and like any beam needs a tower
and a rotor.

Since it is truely frequency independant, for certain uses, like military
that could be operating on any frequency, it is an almost ideal solution.

For hams that are constrained to bands, something like a hex beam
might be a more economical solution.

Your call.

--
Jim Pennino

Lostgallifreyan · October 27th 14, 11:03 AM posted to rec.radio.amateur.antenna

wrote in :

Downside: A high gain antenna can be quite large, require a lot of
expensive aluminum, be quite heavy and like any beam needs a tower
and a rotor.

Since it is truely frequency independant, for certain uses, like military
that could be operating on any frequency, it is an almost ideal solution.

It does sound like it might be viable for portable use, carried in a backpack
to a hilltop. Probably not so good in my back yard though.

Too many
buildings, too many things out there already, 12 solar panels on various
mounts, one tall whip, an FM dipole. I can maybe get an extra longwire out
there but that's about it now.

[email protected] · October 27th 14, 04:49 PM posted to rec.radio.amateur.antenna

Lostgallifreyan wrote:
wrote in :

Downside: A high gain antenna can be quite large, require a lot of
expensive aluminum, be quite heavy and like any beam needs a tower
and a rotor.

Since it is truely frequency independant, for certain uses, like military
that could be operating on any frequency, it is an almost ideal solution.

It does sound like it might be viable for portable use, carried in a backpack
to a hilltop. Probably not so good in my back yard though. Too many
buildings, too many things out there already, 12 solar panels on various
mounts, one tall whip, an FM dipole. I can maybe get an extra longwire out
there but that's about it now.

For VHF/UHF, but not for HF.

You do understand that a log periodic is a series of 1/2 wavelength dipoles
from the lowest to highest frequency of interest?

A "small" log periodic that will cover 20 to 10 M will have a boom about
12 feet long and the longest element will be close to 40 feet long.

--
Jim Pennino

Lostgallifreyan · October 27th 14, 05:43 PM posted to rec.radio.amateur.antenna

wrote in :

You do understand that a log periodic is a series of 1/2 wavelength dipoles
from the lowest to highest frequency of interest?

I do now.

Point taken, this is not a portable thing... I looked very
briefly last night before sleep, my (wrong) assumption was that the elements
were basically directors using a log scale to compress the effective result
into a smaller space... I didn't realise that each was a separate normal
sized 1.2 wave dipole.

[email protected] · October 27th 14, 06:59 PM posted to rec.radio.amateur.antenna

Lostgallifreyan wrote:
wrote in :

You do understand that a log periodic is a series of 1/2 wavelength dipoles
from the lowest to highest frequency of interest?

I do now. Point taken, this is not a portable thing... I looked very
briefly last night before sleep, my (wrong) assumption was that the elements
were basically directors using a log scale to compress the effective result
into a smaller space... I didn't realise that each was a separate normal
sized 1.2 wave dipole.

Well, almost...

The longest element is 1/2 wave at a frequency slightly below the lowest
frequency and the shortest element is 1/2 wave at a frequency slightly
above the highest frequency.

The taper in the element lengths is described by a log function.

The total number of elements and the boom length effect both the gain and
"flatness" of the SWR over the frequency range.

--
Jim Pennino

Lostgallifreyan · October 27th 14, 07:17 PM posted to rec.radio.amateur.antenna

wrote in news

The total number of elements and the boom length effect both the gain and
"flatness" of the SWR over the frequency range.

I can sort of see why the military would like it. Given their resources, it
could be used to rapidly find direction and frequency for many transmissions,
one after another, with no immediate need to co-ordinate a triangulation
effort. Though I suspect that locating sources isn't their primary reason for
using one.

[email protected] · October 27th 14, 07:53 PM posted to rec.radio.amateur.antenna

Lostgallifreyan wrote:
wrote in news
The total number of elements and the boom length effect both the gain and
"flatness" of the SWR over the frequency range.

I can sort of see why the military would like it. Given their resources, it
could be used to rapidly find direction and frequency for many transmissions,
one after another, with no immediate need to co-ordinate a triangulation
effort. Though I suspect that locating sources isn't their primary reason for
using one.

Correct; they are used because the military is not constrained to discreet
bands like hams and it is much simpler to erect one antenna and tower
to cover a wide frequency range, though HF global comm is less and less
used by the military these days as everything is going encrypted space based.

Short range tactical comm still uses a lot of HF, but the antennas there
are usually whips with autotuners at the base.

--
Jim Pennino

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