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Rhombic antenna - getting started.

203.129.50.246
Posted on November 5, 2009 at 21:54:43
Timbo in Oz
Audiophile

Posts: 11394
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
 A tuner driven hard by a low multi-path signal just is the ideal, and I didn't create the specifications of stereo FM, Americans did.

What follows may seem a lot of work and effort for you to do, but if an exterior antenna or a stack, and a rotator and mast, are completely OUT of the question, or too expensive and IF you have the time then a rhombic or two or three, really are the only serious antenna solution available to you. Leaving aside the orientation of your home, and its rooms, and your SO / WAF quotient.

You'll need a map or maps of where you are and where the several stations you REALLY care about are. Should they be at several widely varying compass bearings from your residence, you may well need to make and point more than one rhombic or see*

You'll need to have a copy of the rhombic article. And to have read it, several times IMO&E. * Pay particular attention to the spread-rhombic design - a much wider pattern!? NB It may well soon be hosted here by AA - under FAQ ?

You'll need to have a map / plan of your home's or unit's block with a pointer for North on it. You will need to get a good idea of the directions from inside your residence to each 'desired' station. Draw straight lines from your home to each station's transmitter. Note that the TXERs are not always where the station's studio or building is. FM Fool is one suitable URL for this info. Some of you may live in Seattle where IIRC correctly many/most FM transmitters are up on the needle?

Then, transfer those bearings on to the plan of your home and block. noting that for the usual house paralleled lines for each bearing - until you find a suitable room - are okay as the angles don't shift that much. This is Rule of Thumb (RoT) stuff, okay?

Another way is to buy a single small FM YAGI of 3 to 4 elements maximum and use it to 'find' the bearings and then mark them onto a plan. Don't worry it won't go to waste, you can always point it at a close station ;-).

[Note - you can also use a 'spare' directional antenna to null out an interfering station, you point the spare at the annoyance and then connect it into the cabling, but out of phase with the main antenna or antenna array, so that it cancels that part of that station's coming in from the main antenna (or array) whose aim is to collect some other station / s. The annoyance can be a station right on the same bearing and very strong and close-enough in frequency - but not worth listening to - or one VERY close in frequency but not on the same bearing. Your tuner's front end will struggle less in either case.]

Then you'll need to figure out which rooms are best oriented to fit a large enough rhombic for each given direction. That is, one whose 4 equal sides approach at least one wavelength for that particular station's frequency. (Note that any wire is less fast than a vaccuum so that we SHOULD divide .97 or so into the wavelengths to see how good our match is. An example? at 100Mhz lambda is 3 meters or 9.8425... feet! So the sides need to be about 9.6 feet long.) OTOH don't obsess about it - something that's got sides shorter than lambda will still be directional and still have gain. After all most yagis look to be cut for 98 Mhz and 'adjusted' to do reasonably well at either end of band II.

That is, use the best shaped room for a given direction.

Lofts? If you have a large loft, or more than one -your options get a whole lot better#, as
you can hide them from MOMMY, and
have several much longer ones in one space, and
can keep them pretty fat across so that their pattern isn't too narrow - which makes the bearing issue less critical. And,
in a loft you can use a can of spray paint to mark out the bearings. Last,
if you need a quite narrow pattern for one station and the antenna would be a tad longer, you'll probably have enough room anyway.

[# Notwithstanding the fibre insulation you ought to have up there, above the ceilings. I can advise about how to manage working up there, too! ? Ask if you need to know.]

Where you're going to pin a rhombic to a ceiling rooms you could mark the direction using painters masking tape, as this - shouldn't - remove any existing paint.

Enough for now?










Warmest

Timothy Bailey
The Skyptical Mensurer and Audio Scrounger

And gladly would he learn and gladly teach - Chaucer. ;-)!

'Still not saluting.'

http://www.theanalogdept.com/tim_bailey.htm
You're a fine fellow, Mr. Bailey ..., posted on November 18, 2009 at 11:20:40
rickmcinnis@dogwoodfabrics.com
Audiophile

Posts: 723
Location: Atlanta
Joined: December 15, 2003
 Thanks for the help.

Rick McInnis

RE: Rhombic antenna - getting started., posted on November 6, 2009 at 05:42:32
lahser_3@hotmail.com
Audiophile

Posts: 14
Location: North Carolina
Joined: December 14, 2005
 Here's one


if 3.5db of gain is all you need and a poor pick up pattern, posted on November 6, 2009 at 12:00:49
Timbo in Oz
Audiophile

Posts: 11394
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
 is okay, and how would you know?

It certainly isn't expensive.

But there are plenty of better antennas for sale for not much more.


Warmest

Timothy Bailey
The Skyptical Mensurer and Audio Scrounger

And gladly would he learn and gladly teach - Chaucer. ;-)!

'Still not saluting.'

http://www.theanalogdept.com/tim_bailey.htm
NO it is not. That's a rather small (lambda wise) Vee antenna as a director, with a dipole active element, posted on November 6, 2009 at 11:36:44
Timbo in Oz
Audiophile

Posts: 11394
Location: Canberra - in the ACT - SE Australia
Joined: January 30, 2002
 It isn't a rhombic's bootlace. More below.

As we might expect it has - roughly - the performance of a two element yagi, dipole (active) and director (passive only) and no reflector.

Whereas a single rhombic with 4 (active) sides of length lambda for a given frequency will have 6db of gain, and if you make it from TV ribbon/ 300 ohm cable it will have 12 db gain. Make it longer and the gain goes up

IF you want to know what a rhombic actually is, email me for the article, or ask the moderators here when/if it will be held here at AA.

You can't buy one, you have to make one, and aim it, yourself.

I've been in threads with this company's spokesperson, elsewhere. .......


Warmest

Timothy Bailey
The Skyptical Mensurer and Audio Scrounger

And gladly would he learn and gladly teach - Chaucer. ;-)!

'Still not saluting.'

http://www.theanalogdept.com/tim_bailey.htm

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