Radio Road

The sides are too short.

124.171.147.121

Timbo in Oz

In Reply to: RE: 'Arbitrary' Rhombic? posted by caffeinator on February 14, 2019 at 09:44:48

At the middle of the FM band 98Mhz / 100 Mhz to keep it simple, Lambda is a lot longer then 6 feet, it's more like 10ft.

But you probably do have space for two stacked FM Yagis
and the rotator you may well need.

Warmest

Tim Bailey

Skeptical Measurer & Audio Scrounger

Follow Ups:

RE: The sides are too short. - Triode_Kingdom 19:34:46 02/15/19 (1)
The FM band is so wide as a percentage of frequency, even a yagi is far from ideal. I've never built my own FM antenna, but if I did, it would be a log periodic. Not an easy project, but worthwhile IMO.

--------------------------
Buy Chinese. Bury freedom.
- IF I was to build an antenna it would be the biggest twinned-rhombic I could fit in my loft space! - Timbo in Oz 22:07:32 02/15/19 (0)
  But, it's full of fibreglass, the old original crumbed stuff and two layers of batts.
  I don't need a rotator, and I am only interested in two stations.
  Both come in very quiet and clean from the 7-element yagi which points just East of North.
  I could put a small DIY* 3-element item aimed at the station whose second transmitter is within 1km ESE of us up on a hill. No sign of MP either.
  * I'd cut it for the frequency of 90.3 as well. But I don't need to!
  I feel strongly that a good enough antenna is what people need.
  'Good enough' ? - an antenna that can bring in all your desired stations at full limiting and with minimal multipath.
  In the USA I'd reckon a LPD with a very tight pattern and a good rotator with an indoor indicator, and minimal backlash, just MIGHT be needed.
  But, for people living a good distance from a centre for FM, a big rhombic in their loft might be a lot better.
  Canberra is lucky that we have just three high-up txer sites - the 2nd and 3rd of which are repeaters.
  Some of the stations I'll never bother with are on all 3 and some are on their own.
  
  Warmest
  
  Tim Bailey
  
  Skeptical Measurer & Audio Scrounger
  
  Edits: 02/15/19
RE: The sides are too short. - caffeinator 08:39:16 02/15/19 (2)
Hi Tim,

Thanks! I was hoping you'd chime in. After re-reading the article plus others I found, and especially after surveying the room another time, I realized there just wasn't room for anything near even a 1-Lambda leg length.

Thanks for the Yagi suggestion. Checking Amazon for such, I found one right away that ought to fit. The listing included a user-posted photo of one installed on a garage ceiling - exactly the application I have in mind.

Thanks!

david
- As George Carlin's Spanish priest said 'Hiss hokay man, tre salva Marias!' - Timbo in Oz 20:38:12 02/17/19 (1)
  It may be worth-while for us to discuss yr reception needs.
  
  On reflection, the rhombic idea indicates, that you may not need a rotator, but that you may well need GAIN and a tight pattern.
  
  Are you a good bit away from the transmitters!?
  
  There are a number of ways to stack Yagis. I'm guessing but maybe 4 element or 3 element yagis, for a small space - (and allowing for swing IF needed)
  
  Minimum 6ft / > between their 'planes'.
  
  Try FM Fool to find out what polarity each of your desired stations have, as that can be relevant, and their signal strength / power.
  
  Warmest
  
  Tim Bailey
  
  Skeptical Measurer & Audio Scrounger
  - RE: As George Carlin's Spanish priest said 'Hiss hokay man, tre salva Marias!' - caffeinator 02:33:11 02/18/19 (0)
    Hi again,
    
    There are a lot of stations clustered around 149 deg, 22-25 mi away, Rx (dBm) from -20 to -27. There are two others, both at 184 deg, at -20.5 and -34 dBm, respectively. Those last two are of more interest than the others, though I would like to have access to some of them on occasion.
    
    I took another look at the space and realized I may be able to place an adequately-sized rhombic in the space above the garage doors. Aligning the antenna with the structure would have it pointing toward around 154 deg. The legs would be about 12 feet and the angle about 38 deg.
    
    I'm now leaning back toward the rhombic, since it would be a little less obtrusive than the yagi, and less expensive (nearly free - I have most of the materials).

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