Rhombic for tv reception
Barrett Richardson
tv reception? I've spent a couple of weeks searching at google
for information about the same and haven't found much. The few
reports that say they work great really didn't give much
detailed information. I've put up a small one (10' by 5'10")
and it works, but not fantastic. I'm considering a larger
one, 1 wl on a side for channel 2 -- about 28' total length,
but if I can't get better performance than a consumer grade
Deep Fringe antenna I could probably find a cause more worthy
of the effort. I have a Wineguard pr-9032 which is the best
consumer grade UHF I could find (16 dbd on channel 30) and it
outperforms my scaled down rhombic, and my small rhombic outperforms
the low end VHF/UHF combos (on UHF but not VHF).
-
Barrett
Richard Harrison
"I`ve put up a small one (rhombic) (10` by 5` 10") and it works, but
not fantastic."
It`s too small, Arnold B. Bailey in "TV and Other Receiving Antennas"
has a design I posted here. An Aussie built it and reported it worked as
advertised.
"Description: Four straight horizontal conductors forming a diamond"
"Size at 200 MHz: Two #10 wires each 36 feet long, separated 18 feet at
midpoint, overall length 31 feet"
"Transmission Line: 300 ohms balanced"
"Resistance at Center Frequency: 600 ohms"
Antenna Average Surge Impedance: 600 ohms"
Physical and Patterns: Can`t reproduce here
"Gain at Center Frequency 14.5 db (or less)"
"Frequency Bandwidth: 1 db down, 30%,
3 db down, 100%"
Rhombics are usually 4 or 5 wavelengths on a side at the design
frequency. Bailey`s rhombic is 4.5 wavelengths on a side at 246 MHz. You
could expect more than 14.5 db gain at higher frequencied and less at
lower frequencies. The sides are about one wavelength at channel 2.
Best regards, Richard Harrison, KB5WZI
Richard Harrison
graphics. On the drawing is a 600-ohm termination resistor at the far
end of the rhombic to make the antenna unidirectional in the direction
of the terminated far end. The termination resistor absorbs energy
coming from the direction opposite from the terminated end. Using a
termination resistor may help get a cleaner signal.
Barrett Richardson
On Thu, 21 Feb 2002, Richard Harrison wrote:
> Barrett Richardson wrote:
> > "I`ve put up a small one (rhombic) (10` by 5` 10") and it works, but
> > not fantastic."
>
> It`s too small, Arnold B. Bailey in "TV and Other Receiving Antennas"
> has a design I posted here. An Aussie built it and reported it worked as
> advertised.
>
Do you know if he was able to do any comparison between it and
the best consumer grade deep fringe antennas on the market?
-
Barrett
M.J.Powell
Richardson <bar...@phoenix.aye.net> writes
Have you tried a long wire? Here in UK in the 50s they were used for
fringe reception. One version had a series of small capacitors in the
line, total length about 50 ft for 45Mc/s for London.
Mike
--
M.J.Powell
Richard Harrison
"Do you know if he was able to do any comparison between it and the best
No. He did not say which antennas he had previously tried. He only
reported poor signals before he put up the rhombic and satisfactory
signals after the rhombic was up.
It`s been a year or so, but Bailey`s rhombic was what he said he put up.
The 14 db gain figure versus a dipole (Bailey always uses dbd) is
dependable and can be compared with gain figures for deep fringe
antennas. The deep fringe antennas are likely to have gain quotations at
their best frequency and be referenced to an isotropic which is a couple
of db better looking than dbd. The rhombic figure is at 200 MHz, a
Bailey standard practice. At higher frequencies, the rhombic has higher
gain.
No one has mentioned height, but line of sight to the horizon is about:
D = square root of 2h.
D is the distance to the horizon in miles, and h is the antenna height
in feet. TV broadcasters try to put viewers within their horizon by
using very high transmitting antennas. So, height of the rhombic is
almost unimportant except for getting the vertical pattern directivity
down along the horizontal and not beamed uselessly up towards the sky.
For this, 1/2-wavelerngth of elevation for the rhombic, or more, is
satisfactory. 1/2-wavelength at channel 2 is only about 9 feet. So, that
shouldn`t be too onerous.
J. McLaughlin
(a story unto itself) I spotted a rhombic antenna in a front yard.
Everyone on this group knows how we can spot antennas that can not be
seen by mere mortals. Turned the car around (a task on those roads) and
took a better look. Yep, a miniature rhombic. Could even see a
resistor at the far end from the house. Knocked on the door. Upshot
was that the son-of-the-house had been in the military working with HF
and brought some "learning" home.
It was common to see very long transmission lines from a house to
the antenna that was placed at a hot spot. I am here to tell you that
propagation in such terrain is very interesting.
So, yes. One may and can use a rhombic to receive TV. Not good
value for a radio amateur at HF even if one has the room. 73 Mac
N8TT
--
J. Mc Laughlin - Michigan USA
Home: J...@Power-Net.Net
Office: J...@McLMcL.Com
"Barrett Richardson" <bar...@phoenix.aye.net> wrote in message
news:2002022114363...@phoenix.aye.net...
David Jackson
Antenna Manual, By Woodrow Smith is one.
This one sized for FM radio reception centered on 97 MHz
multiply dimensions by 1.5 for channels 1 to 6
for channels 7 to 13 (highband!?!) multiply dimensions by 0.5
overal length= 55'-9"
sides of diamond = 30'
Distance across center = 22'-6"
minimum mounting height = 10' above obstructions, 30' min, 40' perferred
two 390 ohm terminating resisters
matching section (I can't describe...) but important to 300 ohm twin lead.
> --
> J. Mc Laughlin - Michigan USA
> Home: J...@Power-Net.Net
> Office: J...@McLMcL.Com
> "Barrett Richardson" <bar...@phoenix.aye.net> wrote in message
> news:2002022114363...@phoenix.aye.net...
> > Anybody have experience using small(ish) rhombics for broadcast
> > tv reception? I've spent a couple of weeks searching at google
> > for information about the same and haven't found much. The few
> > reports that say they work great really didn't give much
> > detailed information. I've put up a small one (10' by 5'10")
> > and it works, but not fantastic. I'm considering a larger
> > one, 1 wl on a side for channel 2 -- about 28' total length,
> > but if I can't get better performance than a consumer grade
> > Deep Fringe antenna I could probably find a cause more worthy
> > of the effort. I have a Wineguard pr-9032 which is the best
> > consumer grade UHF I could find (16 dbd on channel 30) and it
> > outperforms my scaled down rhombic, and my small rhombic outperforms
> > the low end VHF/UHF combos (on UHF but not VHF).
> >
> > -
> >
> > Barrett
Santa Rosa, CA
A Macintosh House!
Wes Stewart
The problem with Rhombics is the terrible sidelobes. Without careful design the
first pair can sometimes be bigger than the "main" lobe.
The Laport double parallelogram version can do much better in this regard. I
have modeled one in Eznec that has the largest sidelobes (the thirds) down about
12 dB with F/B >20 dB. Free space gain is about 15 dBi. This is with an
overall length of about 60 feet at 144 MHz or a bit under 9 wavelengths.
Modeling this is tricky because of the multiple wires at the feedpoint and loads
and the number of wires meeting at acute angles and/or crossing. I used segment
tapering on both ends of all wires and then had to tweak slightly to get the
number of segments below 500 (the Eznec limit)
Wes N7WS
Mark Keith
> A full 40 years ago, when doing vhf propagation studies in West Virginia
> (a story unto itself) I spotted a rhombic antenna in a front yard.
> Everyone on this group knows how we can spot antennas that can not be
> seen by mere mortals. Turned the car around (a task on those roads) and
> took a better look. Yep, a miniature rhombic. Could even see a
> resistor at the far end from the house. Knocked on the door. Upshot
> was that the son-of-the-house had been in the military working with HF
> and brought some "learning" home.
Rhombics were very common for TV use back in the 40s,50's. Then the
yagi's and log type designs took over because people wanted to watch
more than one channel and could spin them around. I have a QST article
about TV rhombics somewhere. It was from the 50's I think. A fairly
large "yard sized" rhombic has a lot of gain on VHF. Probably more
than many of todays "TV" antennas being most are compromises to cover
such a large spread of frequencies. MK
Richard Harrison
(Rhombic) Not good value for radio amateur at HF even if one has the
room."
My father-in-law, W1DBM, now SK, would disagree with you. He had 3
rhombics in his favorite directions on his mountain top in Connecticut.
These rhombics excelled in opening bands.
Richard Harrison
"The trouble with rhombics is terrible side lobes."
That`s true with Bailey`s TV rhombic, too. It has side lobes about 40,
50, and 75 degrees either side of the main lobe. If other TV signals on
the same channel are in these directions, they may interfere.
There is significantly more gain in the main lobe than in any side lobe.
So, you`ve improved S/N over an omnidirectional antenna in any case.
Rhombics are good antennas. They are the most often used commercial
antenna for point to point application. This, because they are simple
and work over a wide frequency range without tuning or adjustment.
Rhombics are not ideal, but give good performance over a wider frequency
range.
Richard Harrison
"---matching section (I can`t describe...)---."
A tapered line is the usual matching section for a rhombic. Like the
antenna, iit is not particularly frequency selective.
The spacing at the driven end of the rhombic is chosen to work with the
feedwire size to produce an impedance equal to the characteristic
impedance of the rhombic. Insulators which gradually shrink in length as
they descend in altitude on the feedwires are used to space the feeders.
At the transmission line altitude, the spacing is the same as the
transmission line. It`s a done deal.
A tapered line could have been used by Bailey to transform his specified
600 ohms at the center frequency down to the 300 ohm feedline he
specified. He must have thought it wasn`t worth the trouble. The spacing
between twinlead wires is very small, but larger wire and spacing could
have been used in the downleads. But , the antenna works as specified.
Wes Stewart
Harrison) wrote:
|David Jackson wrote:
|"---matching section (I can`t describe...)---."
|
|A tapered line is the usual matching section for a rhombic. Like the
|antenna, iit is not particularly frequency selective.
|
|The spacing at the driven end of the rhombic is chosen to work with the
|feedwire size to produce an impedance equal to the characteristic
|impedance of the rhombic. Insulators which gradually shrink in length as
|they descend in altitude on the feedwires are used to space the feeders.
|At the transmission line altitude, the spacing is the same as the
|transmission line. It`s a done deal.
Tapered lines are neat. Traveling waves on transmission "don't like" step
changes in Zo (discontinuities); when they encounter them, reflections are
generated. But when the line Zo changes smoothly and gradually, the waves are
"tricked" into thinking all is well.
We used a lot of waveguide tapers when testing Phoenix missile transmitters and
receivers. Because of size constraints, the missile waveguide was 1/4 height
and of course the test equipment was all full-height WR-90.
|
|A tapered line could have been used by Bailey to transform his specified
|600 ohms at the center frequency down to the 300 ohm feedline he
|specified. He must have thought it wasn`t worth the trouble. The spacing
|between twinlead wires is very small, but larger wire and spacing could
|have been used in the downleads. But , the antenna works as specified.
The Laport rhombic I mentioned earlier yields a 300 ohm SWR of under 1.4:1.
Wes Stewart N7WS
Richard Harrison
"A fairly large "yard sized" rhombic has a lot of gain on VHF."
And, it scales to HF.
For shortwave broadcasting, we might be ordered to transmit to a new
country, or on a band for which we had no curtain antenna, or on a band
in which all our curtains were in use at the scheduled times.
We coukd throw up a rhombic in a few hours and blister the new target
with signal. A nice feature was being able to use the rhombic on any
frequency between 6 and 17 MHz without touching any antenna tuning
adjustments. Yes, the width of the target covered shrank as frequency
rose. Yes, more of the signal overshot the target as frequency rose.
But, as Cecil observed, halitosis is better than no breath at all.
Sometimes, we even used the rhombic outside the usual 6 to 17 MHz range,
and they still worked.
For program relays, we used only rhombics on both ends of the circuit.
There were just too many frequency changes needed to keep on top of
changing propagation to always get the best signal. We used triple
frequency diversity and were perpetually shifting to lower or higher
frequencies. The order wire was busy saying: put this language or that
language on the next higher or lower frequency, that is, one of the
three frequencies simultaneously carrying the program relay of that
language at the moment.
For broadcasts you only make scheduled program changes. You must
announce your schedules over and over in advance so listeners will know
where to look for you. The schedules are based on propagation
predictions and you have to be there whether there is propagation or no
propagation because these conditions can change rapidly. The curtain
antenna can serve well. You will be broadcasting for hours on the same
frequency. You can adjust matching networks in advance of frequency
changes when another curtain will resume service. That doesn`t work well
for program relay.
Barrett Richardson
On Thu, 21 Feb 2002, Richard Harrison wrote:
> No one has mentioned height, but line of sight to the horizon is about:
> D = square root of 2h.
> D is the distance to the horizon in miles, and h is the antenna height
> in feet. TV broadcasters try to put viewers within their horizon by
> using very high transmitting antennas. So, height of the rhombic is
> almost unimportant except for getting the vertical pattern directivity
> down along the horizontal and not beamed uselessly up towards the sky.
> For this, 1/2-wavelerngth of elevation for the rhombic, or more, is
> satisfactory. 1/2-wavelength at channel 2 is only about 9 feet. So, that
> shouldn`t be too onerous.
>
This may be some good news. An example of a station I'm after is
15.2kw antenna 78 meters above ground 72 miles away on UHF
channel 27. For comparison I can get a fuzzy unwatchable picture
picture from a channel 50, 26.2kw at 100 meters, 55 miles away
with the smallest UHF/VHF combo Radio Shack sells. The FCC
data for the channel 50 shows a field value of about 0.3 in
my direction. The Wineguard pr-9032 gives only a marginal
improvement on channel 50. With the Wineguard I can also ascertain
there is a channel 27 there, but this may be deceptive because
there is a much stronger channel 27 about 100 miles distant to
the rear. On the same tower as the channel 50 and about the
same height there is a channel 24 with 10.9 kw but a 398kw
channel 24 is about 45 miles to the rear. The stronger channel
24 to the rear disappears with the 10ft rhombic pointed opposite,
but I still can't get the other channel 24. My small Radio Shack antenna
and Wineguard both just get the channel 24 that is to the rear,
so the f/b ratio of a rhombic helps in that regard. I had blamed the
mediocre improvements of the Wineguard's performance on lack of height
(it's only at twelve feet) and was in the process of figuring out how to
get a larger temporary rhombic 30 ft off the ground for testing without
lots of labor. If I can worthwhile information by testing for UHF
reception just a few feet off the ground, that is definitely good news.
-
Barrett
Barrett Richardson
On Thu, 21 Feb 2002, M.J.Powell wrote:
> Have you tried a long wire? Here in UK in the 50s they were used for
> fringe reception. One version had a series of small capacitors in the
> line, total length about 50 ft for 45Mc/s for London.
>
I found a couple of test posts at google that mentioned it,
but couldn't find any details.
-
Barrett
Richard Harrison
lower frequency."
This was a request from the receiver site to the transmitter site to
change one of the three frequencies carrying a program relay. I
insinuated that propagation caused changes, but that`s not all. The
spectrum is sometimes an undiscplined place, so the receiver site checks
the repertoire for a clean frequency that can work when interference
alights as it too frequently does. Three viable frequencies are needed
at all times so that the triple diversity reception works properly.
Richard Harrison
"This may be some good news. An example of the station I`m after is 15.2
kw antenna 78 meters above ground 72 miles away."
The news could be better. Do you have an obstructed path? A mountain
between you and the transmitter?
78 meters above a "plane" earth surface gives a horizon of about 22
miles. Your horizon needs to be 50 miles to total 72 miles. Your antenna
would only have to be up 1250 feet if there were no path obstructions.
The atmosphere bends and scatters radio and TV signals but reliable over
the horizon transmission requires huge antenna gains on both ends of a
path in most cases.
TV receives a wide band, up to 6 MHz. More signal is needed to compete
with noise in so much bandwidth. Radio usually occupies less bandwidth
and takes less signal for satisfactory reception.
You will have anomalous propagation which provides occasional long
distance reception, but don`t count on it.
Barrett Richardson
On Fri, 22 Feb 2002, Richard Harrison wrote:
>
> The news could be better. Do you have an obstructed path? A mountain
> between you and the transmitter?
No, but I take the height comment only applies to line of sight and
not the forward scatter phenomenom.
>
> 78 meters above a "plane" earth surface gives a horizon of about 22
> miles. Your horizon needs to be 50 miles to total 72 miles. Your antenna
> would only have to be up 1250 feet if there were no path obstructions.
Really wasn't goint to attempt line of sight. Get 15 stations clear
now, have line of sight on only two of them.
>
> The atmosphere bends and scatters radio and TV signals but reliable over
> the horizon transmission requires huge antenna gains on both ends of a
> path in most cases.
>
> TV receives a wide band, up to 6 MHz. More signal is needed to compete
> with noise in so much bandwidth. Radio usually occupies less bandwidth
> and takes less signal for satisfactory reception.
>
> You will have anomalous propagation which provides occasional long
> distance reception, but don`t count on it.
>
Getting the weak channel 50 from so far beyond it's horizon on a
continuous basis just got me wondering if I could snag a couple
more about 15 miles farther out. If such a thing exists, I think
I'm in kind of a local sweet spot. I've always gotten better
reception than others in the locality and have a smaller and
shorter antenna.
-
Barrett