to appear as about 800 ohms and then the spacing tapered steadiy down
during the descent to the horizontal transmission line which was spaced
for 600 ohms.
I like the tapered arrangement. What's the minimum distance needed to
taper from 800 to 600 at freqs from say 3.5MHz to 14MHz? I did one years
ago from 130 to 200 and it was about a foot long at 1.8GHz.

Best regards, Richard Harrison, KB5WZI


Alan

On Sun, 01 Oct 2006 17:27:31 +1000, Alan Peake
wrote:



to appear as about 800 ohms and then the spacing tapered steadiy down
during the descent to the horizontal transmission line which was spaced
for 600 ohms.
I like the tapered arrangement. What's the minimum distance needed to
taper from 800 to 600 at freqs from say 3.5MHz to 14MHz? I did one years
ago from 130 to 200 and it was about a foot long at 1.8GHz.

Alan,

Are you tapering for impedance transformation, or just to reduce
transmission line losses?

I make the matched line loss on a 600 ohms open wire line with 2mm dia
copper spaced 150mm something approaching 0.1dB/100m, with 2:1 VSWR
you are talking ~0.14dB/100m. You should find the rhombic no worse
than 2:1 unless it is too short.

My loss calculator has an ideal air spaced 600 ohm copper line (copper
loss only) 2mm/150mm, the label is "Open / air dielectric (150/2.00)",
you can try the various loads from the NEC models for more exact loss
figures.

Owen
--

On Fri, 29 Sep 2006 16:56:07 +1000, Alan Peake
wrote:

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?
Alan
VK2ADB

The rhombic can deliver you a frequency agile antenna with gain, and
low angle major lobe if of sufficient length and at sufficient height.
Side lobes are not pretty, space requirements are huge at HF and the
antenna is not readily rotatable, construction is simple, but serious.

You are on hectares (doesn't sound as good as acres, does it?). Space
is not a big issue, and every ham that can accomodate a good size
rhombic should have one (or more) as a talking point. You could deal
with the fixed heading disadvantage two ways: place the shack in the
middle of the rhombic and switch feed / load ends, or go the whole hog
and erect a set of rhombics to cover your desired / preferred paths.

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m. Sag of 5% of
span is easily accomodated if the end heights are at 20m of more, but
becomes a problem as you lower the end height much. You could model
the effect of the combination of sag and low end height in NEC by
breaking the leg wires into several sections following the approximate
catenary (or parabola for ease). I haven't done it, but I suspect
uncertainty about the ground conditions and ground profile will
introduce more model error than modest sag.

Owen
--

Owen Duffy wrote:
On Fri, 29 Sep 2006 16:56:07 +1000, Alan Peake
wrote:

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?
Alan
VK2ADB

The rhombic can deliver you a frequency agile antenna with gain, and
low angle major lobe if of sufficient length and at sufficient height.
Side lobes are not pretty, space requirements are huge at HF and the
antenna is not readily rotatable, construction is simple, but serious.

You are on hectares (doesn't sound as good as acres, does it?). Space
is not a big issue, and every ham that can accomodate a good size
rhombic should have one (or more) as a talking point. You could deal
with the fixed heading disadvantage two ways: place the shack in the
middle of the rhombic and switch feed / load ends, or go the whole hog
and erect a set of rhombics to cover your desired / preferred paths.

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m. Sag of 5% of
span is easily accomodated if the end heights are at 20m of more, but
becomes a problem as you lower the end height much. You could model
the effect of the combination of sag and low end height in NEC by
breaking the leg wires into several sections following the approximate
catenary (or parabola for ease). I haven't done it, but I suspect
uncertainty about the ground conditions and ground profile will
introduce more model error than modest sag.

Owen
--

Dollar for Dollar DB for DB i would errect a Curtain antenna. It
outperforms Rhombics in all aspects of design and construction.
Considering that the 4 poles of a Rhombic can be used to build an
Array that will cover the globe, have equal or greater gain that would
make the curtain antenna a better choice. I suppose thats why just
about ever shortwave station in the world uses them!

If you want an opinion of a station who has used and is still using
stacked rhombics ask Ian VK3MO. He will tell you thats his 6 wavelength
perside rhombic is a terrible general purpose antenna. Since his
antenna is fixed on new York he finds that quite frequently the
propagation path rarely comes in on the direct computed bearing. He
loses 10 to 20 db having his sharp rhombic. His rhombic does not have
sufficient azimuth diversity since the 3db horizontal beamwidth is so
narrow.

You also wont have the problem of finding a termination resistor, a
decent globar one anyway. See w8ji.com or look at TCI's web page. Its
one hell of a antenna. I am surprised nobody in ham radio has installed
one, considering the large number of 200 ft towers in the world. When
radio Switzerland closed down there was a special even ham station on
air using a rotable 300ft high curtain, oh what a signal 24 dbi
produces with a 100 watts! Now if you know of a stacked array that uses
4 mono band or other stacked antennas that cal deliver greater than
20dbi thats not a laser beam let us all know. I tried to come with
such a stack using 6 log periodics using 42 ft booms on a 200 foot
tower. It deliver such gain on the higher frequencies but fell short on
13mhz. The curtain will do it with a lot less expense.

Bob



Bob

Dollar for Dollar DB for DB i would errect a Curtain antenna. It
outperforms Rhombics in all aspects of design and construction.

Well, I don't know much about curtains but VK3ATN apparently had one up
and preferred his rhombics. Don't know the full details though.


If you want an opinion of a station who has used and is still using
stacked rhombics ask Ian VK3MO. He will tell you thats his 6 wavelength
perside rhombic is a terrible general purpose antenna. Since his
antenna is fixed on new York he finds that quite frequently the
propagation path rarely comes in on the direct computed bearing. He
loses 10 to 20 db having his sharp rhombic. His rhombic does not have
sufficient azimuth diversity since the 3db horizontal beamwidth is so
narrow.

That's a problem, which is why I had thought of only 4 wavelengths per
side. NEC says it would be about 15 degrees wide on 20m.


You also wont have the problem of finding a termination resistor, a
decent globar one anyway.

I was going use it unterminated to begin with. Aimed at Europe, the
other end points at Central America so I'd be surprised if I had
problems covering both those areas at the same time.

Alan

Keeping in mind your exposure to high winds and snow (ice loading),
the construction needs to be robust. If for example you want coverage
down to 20m you should be aiming legs of close to 100m.

Yes, it looks like 80m per leg is reasonable - 4 wavelengths at 20m
I did as you suggested in terms of modelling the sag and it didn't seem
to upset the pattern greatly.

I can put the antenna over the house but I'm not sure if the extra
feeder loss would outweigh the advantages. Might not be too bad with 600
ohm or greater feeder.
Alan

Alan Peake wrote in
:

Before I go to the trouble of putting up a rhombic, I've been using NEC
to get an idea of the gain, radiation angle etc for various leg lengths.
It all looks very promising on the computer but I'd be interested in
real-world experiences. For example, how well does the real antenna
approach the PC simulation when various factors like wire sag, uneven
ground, presence of trees and shrubbery?

I only ever had the use of one REAL rhombic. And that was on 60m in the
high arctic (gov't freqs). It was properly designed for the specific path
we were using and yes, it was VERY good. You could almost make your own
band opening with 5kw!

Properly designed, they have a good reputation for doing what the theory
says they will do. Just remember, though, that you're going to have to
sewer almost half your transmitted power into the terminating resistor.
But that's the half that would be going the wrong way, basically.


--
Dave Oldridge+
ICQ 1800667

Dave Oldridge wrote:

Properly designed, they have a good reputation for doing what the
theory says they will do. Just remember, though, that you're going to
have to sewer almost half your transmitted power into the terminating
resistor. But that's the half that would be going the wrong way,
basically.

But it would get there - eventually Long or short path. But I don't
know if both paths are ever open at the same time to the same extent. If
not, then it may not be a problem. What about running a transmission
line from where the terminating R would be, back to the feed point?
Assuming you can match it all that is.
Alan

Alan Peake wrote:


Dave Oldridge wrote:

Properly designed, they have a good reputation for doing what the
theory says they will do. Just remember, though, that you're going to
have to sewer almost half your transmitted power into the terminating
resistor. But that's the half that would be going the wrong way,
basically.

But it would get there - eventually Long or short path. But I don't
know if both paths are ever open at the same time to the same extent.
If not, then it may not be a problem. What about running a transmission
line from where the terminating R would be, back to the feed point?
Assuming you can match it all that is.

No problem. Pipe the signal back from the far end into the shack, feed
it into a circulator, and add it to the outgoing signal. Cecil will
explain what happens to the power :-)


Replying to Yuri's point: from personal experience of using a rhombic
100 wavelengths long for 2m moonbounce, it had only about the same
maximum gain as a box of 4 mid-size yagis - and that is only while the
moon is passing through the very narrow main beam, which only happens
for a magic 20 minutes on certain days of the month.

In other words, the rhombic did work, but the performance was nowhere
near as spectacular as we had expected from its huge electrical length.

What is undeniably true is that it *looked* spectacular! I've used many
kinds of antennas since then, up to an 85ft dish, but not one of them
has given me the same buzz as that rhombic. And there is the trap: buzz
isn't the same thing as performance.

We need to be very careful about applying dual standards. An unavoidable
feature of all very long rhombics is that the main beam is very narrow,
because the edges of the main lobe are sliced away by large numbers of
sidelobes that are not many dB down. If we saw that kind of E-plane
pattern in a yagi, we wouldn't hesitate to call it a "bad design"... so
what's "good" about the same feature in a rhombic?



--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:
Pipe the signal back from the far end into the shack, feed
it into a circulator, and add it to the outgoing signal. Cecil will
explain what happens to the power :-)

Dr. Best, VE9SRB, in his 2001 QEX articles explained what
would happen. Based on his idea that 75w + 8.33w = 133.33w,
one could route the unused Rhombic power back to the source,
recycle it, and cause voltage superposition to multiply the
power up to a factor of 4. :-) To anyone who thinks I am kidding
about Dr. Best posting the above equation, it can probably
be verified by Google circa May 2001 on this newsgroup.

Dr. Best proved his assertions with the following power equation:

Ptot = P1 + P2 + 2[SQRT(P1*P2)]cos(A)

where A is the angle between V1 and V2. If we make the angle
between V1 and V2 equal to zero, we can take the P1 power
from the source and the P2 power routed back from the load
and increase our total power output by a factor equal to
2[SQRT(P1*P2)]. Who says there is no such thing as a free
lunch? :-)

Discussed by me in May 2001 was the fact that his term,
2[SQRT(P1*P2)] is constructive interference energy which must
necessarily be exactly balanced by 2[SQRT(P1*P2)] watts of
destructive interference energy or else the conservation of
energy principle is violated. At the time, Dr. Best did not
understand where the necessary destructive interference energy
was coming from. It comes from the Z0-match between the feedline
and the source and it works exactly like the thin-film layer on
non-reflective glass. Dr. Best's Ptot equation above is true for
A = 0 if and only if Ptot is being supplied with destructive
interference energy where A is probably equal to 180 degrees.
--
73, Cecil http://www.w5dxp.com