Rhombic UHF TV antenna question

[JasonW]

Hello All,

I'm looking into building a rhombic UHF TV antenna for a friend of mine who lives in an outdoor antenna-restricted neighborhood. Fortunately he can receive all of the local VHF and UHF TV stations well with indoor antennas, except for one distant UHF station (it comes in, but weakly with "snow").

The rhombic I'm thinking about would be made of wire, mounted to the ceiling and pointed at the station. Making it 12' - 15' long (several wavelengths at UHF frequencies) should give it gain, and terminating its far end would make it unidirectional with a broad-banded response.

My question is: Will it work if I use a 300 ohm terminating resistor (1/2 watt carbon) and feed the rhombic with either 300 ohm twin-lead or a 75 ohm:300 ohm feedpoint balun with 75 ohm coax to the TV set? All of the rhombic antenna literature I've read says that they're fed with 600 ohm open wire line and terminated with 600 ohm resistors, but I've wondered if that's because telephone poles with 600 ohm balanced telephone wire pair spacing (on the cross timbers) may have been used for supporting the feedlines of early Short Wave broadcasting rhombics.

Many thanks in advance to anyone who can help. -- J. Jason Wentworth

 

[John_U]

> Hello All,
>
> I'm looking into building a rhombic UHF TV antenna for a
> friend of mine who lives in an outdoor antenna-restricted
> neighborhood. Fortunately he can receive all of the local
> VHF and UHF TV stations well with indoor antennas, except
> for one distant UHF station (it comes in, but weakly with
> "snow").
>
> The rhombic I'm thinking about would be made of wire,
> mounted to the ceiling and pointed at the station. Making
> it 12' - 15' long (several wavelengths at UHF frequencies)
> should give it gain, and terminating its far end would make
> it unidirectional with a broad-banded response.
>
> My question is: Will it work if I use a 300 ohm terminating
> resistor (1/2 watt carbon) and feed the rhombic with either
> 300 ohm twin-lead or a 75 ohm:300 ohm feedpoint balun with
> 75 ohm coax to the TV set? All of the rhombic antenna
> literature I've read says that they're fed with 600 ohm open
> wire line and terminated with 600 ohm resistors, but I've
> wondered if that's because telephone poles with 600 ohm
> balanced telephone wire pair spacing (on the cross timbers)
> may have been used for supporting the feedlines of early
> Short Wave broadcasting rhombics.
>
> Many thanks in advance to anyone who can help. -- J. Jason
> Wentworth
>
Hi,

A beter choice, unless you are interested in building antennas is to buy a UHF Yagi cut to the frequency of interest and mount that to the ceiling. Another choice is to look up "stealth" antennas. Many amateur radio operators with CCR problems use "stealth" antennas. You might want to make a UHF version. Getting higher and outside makes a huge difference. And depending on how much you want to invest, you might just think about getting the digital feed with a converter box. If it is just snow and not ghosting, digital may work wonders.

John

 

[jakarta_mark]

> > Hello All,
> >
> > I'm looking into building a rhombic UHF TV antenna for a
> > friend of mine who lives in an outdoor antenna-restricted
> > neighborhood. Fortunately he can receive all of the local
>
> > VHF and UHF TV stations well with indoor antennas, except
> > for one distant UHF station (it comes in, but weakly with
> > "snow").
> >
> > The rhombic I'm thinking about would be made of wire,
> > mounted to the ceiling and pointed at the station. Making
>
> > it 12' - 15' long (several wavelengths at UHF frequencies)
>
> > should give it gain, and terminating its far end would
> make
> > it unidirectional with a broad-banded response.
> >
> > My question is: Will it work if I use a 300 ohm
> terminating
> > resistor (1/2 watt carbon) and feed the rhombic with
> either
> > 300 ohm twin-lead or a 75 ohm:300 ohm feedpoint balun with
>
> > 75 ohm coax to the TV set? All of the rhombic antenna
> > literature I've read says that they're fed with 600 ohm
> open
> > wire line and terminated with 600 ohm resistors, but I've
> > wondered if that's because telephone poles with 600 ohm
> > balanced telephone wire pair spacing (on the cross
> timbers)
> > may have been used for supporting the feedlines of early
> > Short Wave broadcasting rhombics.
> >
> > Many thanks in advance to anyone who can help. -- J.
> Jason
> > Wentworth
> >
> Hi,
>
> A beter choice, unless you are interested in building
> antennas is to buy a UHF Yagi cut to the frequency of
> interest and mount that to the ceiling. Another choice is to
> look up "stealth" antennas. Many amateur radio operators
> with CCR problems use "stealth" antennas. You might want to
> make a UHF version. Getting higher and outside makes a huge
> difference. And depending on how much you want to invest,
> you might just think about getting the digital feed with a
> converter box. If it is just snow and not ghosting, digital
> may work wonders.
>
> John
>
Hi Jason,

The rhombic "wants" to see a 600 ohm termination because that
is the way it was designed, not because of some arbitrary telephone
wire spacing.

John's advice above, about making a "stealth" outside antenna, is
very good. With relatively thin wires, you can make an outside
antenna that does not look like an antenna. If you must stay inside,
a more modern antenna, like a computer optimized Yagi, or maybe
even a Quagi, might net you more gain and a better pattern, if
rejection of any co-channel stations is a consideration. Back to the
rhombic, you would need to terminate it with 600 ohms and, ideally,
use an 8:1 balun to get down to 75 ohms. Dr. Jerry Sevick has
written several books on Baluns and Ununs - perform a Google
search to get that book information. If you don't have the correct
ratio balun, you may be able to get usable signal with a 6:1 or
say, 10:1 (if you can obtain one [not common ratios]) or wind your
own, with some standing wave on the line.

One more antenna source might be the "Cheap Yagi" by Kent
Britain (WA5VJB) - although I don't know if he has noodled out
one for your specific UHF TV channel.

Good luck,

- Mark

 

[JasonW]

I thank you all very much for your replies. The rhombic has another non-engineering advantage in this situation--it has his XYL's (wife's) approval!

I like Dr. Sevick's baluns and ununs. Using one of his 12:1 baluns at the rhombic's feedpoint (if it'll work at UHF frequencies--I'll have to check the specs) connected to one of his 50 ohm:75 ohm ununs running to the TV set should work well. -- Jason

 

[CW]

> Hello All,
>
> I'm looking into building a rhombic UHF TV antenna for a
> friend of mine who lives in an outdoor antenna-restricted
> neighborhood. Fortunately he can receive all of the local
> VHF and UHF TV stations well with indoor antennas, except
> for one distant UHF station (it comes in, but weakly with
> "snow").
>

NOONE can live in an outdoor TV antenna restricted neighborhood anymore!
With the FEDERAL OTARD law in effect, you can put up an outside antenna of reasonable size and height to pick up over the air TV signals...
If the Neighborhood NAZI group (my name for HOAs) need convincing, they can read it for themselves on the FCC web site..
Have him go buy some Rohn 25G and the BIGGEST Channel Master antenna and go for it...and tell the NAZI group to shove it

Read the facts at:

http://ftp.fcc.gov/cgb/consumerfacts/consumerdish.html

 

[CW]

> Hi,
>
> A beter choice, unless you are interested in building
> antennas is to buy a UHF Yagi cut to the frequency of
> interest and mount that to the ceiling. Another choice is to
> look up "stealth" antennas. Many amateur radio operators
> with CCR problems use "stealth" antennas. You might want to
> make a UHF version. Getting higher and outside makes a huge
> difference. And depending on how much you want to invest,
> you might just think about getting the digital feed with a
> converter box. If it is just snow and not ghosting, digital
> may work wonders.

A stealth antenna is no longer needed for TV reception thanks to Federal law....besides, inside antennas have a 12db loss or more compared to outside antennas...all other things being equal...put up an outside TV antenna (and if he is a ham, I would use one of the 50-1200Mhz LP antennas...and put a switch inline so he can use it on the ham bands

http://www.fcc.gov/mb/facts/otard.html

73
-.-. .--

 

[JasonW]

Unfortunately, there are sometimes situations in life where, even though you *can* do something, it isn't worth the trouble to do so. My friend is elderly and in fragile health, and even though he is now (thanks to the FCC) perfectly within his rights to put up an outdoor TV antenna, the trouble and stress the confrontation would cause isn't worth it to him. (Hams who by law don't *have* to help their neighbors with RF interference problems often do so to keep peace in the neighborhood.)

The indoor UHF rhombic TV antenna will avoid causing problems with his neighbors. -- J. Jason Wentworth

 

[CW]

> Unfortunately, there are sometimes situations in life where,
> even though you *can* do something, it isn't worth the
> trouble to do so. My friend is elderly and in fragile
> health, and even though he is now (thanks to the FCC)
> perfectly within his rights to put up an outdoor TV antenna,
> the trouble and stress the confrontation would cause isn't
> worth it to him. (Hams who by law don't *have* to help
> their neighbors with RF interference problems often do so to
> keep peace in the neighborhood.)
>
> The indoor UHF rhombic TV antenna will avoid causing
> problems with his neighbors. -- J. Jason Wentworth

All he needs to do is show them the law...a lot of times, they dont know it exists AND will be interested to know THEY can put up outside antennas..
if the HOA cannot do anything, neither can his neighbors...when I lived in a neighborhood that was similar, I put up TV and ham antennas anyway...noone complained to me and the HOA never came and complained...
Putting up a TV on a 20ft push up mast (or pipe which is easy to do..I have my Dish Network dish on just two sections of 2in pipe screwed together and attached to the eave via a clamp) is simple and easy and will work MUCH better than an inside antenna...remember the 12db difference...it IS worth it.

 

[Tom_Desmond]

> My question is: Will it work if I use a 300 ohm terminating
> resistor (1/2 watt carbon) and feed the rhombic with either
> 300 ohm twin-lead or a 75 ohm:300 ohm feedpoint balun with
> 75 ohm coax to the TV set? All of the rhombic antenna
> literature I've read says that they're fed with 600 ohm open
> wire line and terminated with 600 ohm resistors, but I've
> wondered if that's because telephone poles with 600 ohm
> balanced telephone wire pair spacing (on the cross timbers)
> may have been used for supporting the feedlines of early
> Short Wave broadcasting rhombics.

The reason that you're using the 600 ohm resistors is that there are two of 'em in parallel, and two paralleled 600 ohm resistors gives you an equivalent impedance of 300 ohms. That matches the once traditional 300 ohm impedance that was common for TV when twinlead was the normal cabling used.

So, yeah, you'll need to feed it with 300 ohm twin lead or use a 75 ohm:300 ohm balun. Most likely, you'll need to do the latter, since 300 ohm inputs seem to be getting pretty rare (unless your friend has a really ancient television).

 

[JasonW]

His TV set has a 75 ohm input. I'll run 300 ohm twin-lead from the rhombic feedpoint to the set, connecting it using a 75 ohm:300 ohm balun screwed to the set's "F" connector. -- Jason

> > My question is: Will it work if I use a 300 ohm
> terminating
> > resistor (1/2 watt carbon) and feed the rhombic with
> either
> > 300 ohm twin-lead or a 75 ohm:300 ohm feedpoint balun with
>
> > 75 ohm coax to the TV set? All of the rhombic antenna
> > literature I've read says that they're fed with 600 ohm
> open
> > wire line and terminated with 600 ohm resistors, but I've
> > wondered if that's because telephone poles with 600 ohm
> > balanced telephone wire pair spacing (on the cross
> timbers)
> > may have been used for supporting the feedlines of early
> > Short Wave broadcasting rhombics.
>
> The reason that you're using the 600 ohm resistors is that
> there are two of 'em in parallel, and two paralleled 600 ohm
> resistors gives you an equivalent impedance of 300 ohms.
> That matches the once traditional 300 ohm impedance that was
> common for TV when twinlead was the normal cabling used.
>
> So, yeah, you'll need to feed it with 300 ohm twin lead or
> use a 75 ohm:300 ohm balun. Most likely, you'll need to do
> the latter, since 300 ohm inputs seem to be getting pretty
> rare (unless your friend has a really ancient television).
>