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In Reply to: RE: how many have tried a Rhombic FM Antenna? posted by user510 on June 13, 2017 at 13:39:36
The opening post does indicate the use of a rhombic antenna. However, before getting tooled up for that, I thought to optimize use/position of that automotive whip antenna I bought for $10 last week. I invest this much effort because I anticipate some potential good use.
To mount the antenna to the small tripod pictured, I took a piece of 1/4" thick acrylic plate, drilled two holes, one for the 1/4-20 threaded bolt fastener and one for the antenna base to fasten into.
Now I can raise/lower the height of it. The tripod also allows rotation on two planes, horizontal and vertical.
I find that with some experimentation with the rotation, altitude and x,y, positioning, I have a lower noise floor. At the moment I am not using the multipath reduction switch on the tuner. And this is an improvement in noise levels from the previous mounting. Signal strength remains the same. 4 lights.
edits/updates to this post:
I have noticed that throughout the day reception clarity/noise does change. It changes when the atmosphere becomes cloudy (better) and when it transitions from overcast into cloudless blue sky conditions (not always as good). It changes from early morning to late afternoon even when the weather remains constant throught the day. Perhaps reception is best and cleanest sounding in the evening/early morning hours.
I have seen similar effects in all of the various places I have lived throughout my life. However, I have rarely been able to use an outdoor FM antenna, as it would be preferred.
They are supposed to have more gain.
Mount it on a vertical board and put a 'lazy susan', under it to turn it to point it at stations.
You can make your own and cut them to a 1/4 wavelength of a particular station, divide by .95 IIRC.
Skeptical Measurer & Audio Scrounger
yep. See link below to the post in this thread.
I have always heard the standard, closed loop "T", which is included free with so many tuners to be called a folded dipole. I see one on the wall in the OP photos 2 and 4.
Timbo I believe you know much more about this and other electronics than I do, so I'm surprised to see you recommend it when I thought they have possibly the minimal gain of any antenna type.
"The piano ain't got no wrong notes." Thelonious Monk
Because they are directional, Steve should be able to get some in-room bearings, and also eliminate a few (a lot of?) stations from his wanted list.
Then he can consider a spread rhombic, or one or two, laid on top of each other, and a switch.
Because most stations use mixed polarity, it should be okay to use a vertical dipole to search, and install horisontal rhombics on the ceiling that point in the directions he finds that work.
Skeptical Measurer & Audio Scrounger
It would be my understanding that the main element in the Yagi is the folded dipole.
Am I wrong to assume that, like the automotive whip antenna, and the 1/2-wave dipole up on the wall, a folded dipole, without any other elements added to the stick, is also omni-directional?
I might try one.
some idle ideas.
I haven't given up on the idea of the Rombic. I could devise one that would circumscribe the dimensions of my listening room. Mount it up high just under the ceiling. It would be somewhat imposing to behold, so to speak, but it is directional. Aiming a Rombic would be complicated when the thing needs to be attached somehow, or hung somehow, from the upper portions of the walls.
It would be simple and inexpensive to buy a 1/2 wave length telescopic whip to construct my own version of the Magnum Dynalab ST-2 FM antenna. Only instead of 54 inches long I'd use a telescopic rod with a max length of at least 72 inches. 1/2 wave. The thing about the ST-2 that attracts me is that it just 'looks cool'. I doubt it would be any more effective than the 1/4 wave whip I'm using at the moment. But a 1/2 wave version would have more gain.
Yet it would still be susceptible to the interference I'm experiencing with what I have now. And I wonder if a Rhombic would also be susceptible in the same way.
I could diy a full wavelength dipole. Even more gain. But signal strength isn't the problem I face, really. Though it would be nice to pull in a couple of stations being broadcast from Portland. I used to live there and had some favorite stations back in those days. Portland is about 90 miles from my location so that would stretch the limit.
Now, having lived with the two antennas I have in use for a month or so I wonder if my thinking about atmospheric conditions having an effect are just hogwash. What if I am experiencing interference from the surrounding electrical systems within the building I'm in. let's say some neighbor in the same building turns on an appliance and the white noise appears within my FM reception. Maybe when people get home from work around 5pm and start running their appliances I get more white noise. Some days it seems like that. But not every day.
I'm not giving up. I have taken a step back in the past week and concentrated on other stuff on my agenda. I'll return to the FM reception issue fairly soon.
Folded dipole. How large should I make it?
Ps: I should note that the 'noise' issue I face is not constant. It comes and goes. At the moment I'm listening to my target FM station and receive a quiet clean signal. Good listening ..... for now. I know it will change. The noise will come back. I just haven't identified the cause of the noise.
The axis is at right angles to the two wires in the 300 ohm ribbon.
Their polar diagram is a figure 8. Like a figure-8 mike. The cross-section .. of the two wires is inside the two lobes of the polar diagram. IE <-OO-> . Okay?
So that, when pinned flat to a narrow board aligned vertically, you can rotate it along that vertical axis it so that it points at the strongest signal in that room, for each station. A lazy Susan type rotating base is the most convenient.
Noting that most FM transmitters use mixed polarisation IE 50% vertical and 50% horisontal - a vertical dipole on a board with a rotating base is more practical than trying to orient a horisontal dipole to maximum signal.
I'd recommend that you at least twist the down-lead part of the ribbon to minimise its signal pick-up capacity. OR buy a 300/750hm balun and run coax to the tuner.
You could also add another dipole on the other side and run them in parallel. But you would need to wind a 150:75 ohm balun. But even that stuff is on the web! :-)
Hope this helps.
Once you can get reasonable signal strength on most stations you can begin mapping your preferred station's axes on to yr dwelling's plan.
Then you might have enough information to decide if you want a Rhombic or Rhombics. And, where to point them.
Retail ribbon dipoles are cheap. OTOH Plans of dipoles should be available on the web. 300 ohm ribbon has IIRC a propogation delay of about 5% so the 1/4 wave length for a given frequency in Mhz, should be divided by .95 which will make it a bit longer so that it does resonate at the right freq.
I suggest you either build one that's as long as you'ld need for 108Mhz OR just buy one and learn with it. Those are usually cut for ~98 Mhz.
Skeptical Measurer & Audio Scrounger
Thanks for posting - the tripod is a great idea for experimentation like this. I'll shamelessly copy that so I can do some similar experiments for FM and HD antennae I've been trying to evaluate and orient optimally.
Btw, when you were closer to Seattle, what sort of antenna did you find worked best?
"Btw, when you were closer to Seattle, what sort of antenna did you find worked best?"
I was living in Lynnwood for 16-1/2 years. Lots of available FM stations to tune in. In my area multipath seemed the biggest problem. And atmospherics played into it. Some days the dipole I had up high on the south wall, above the sliding door, was cleanest. Other days, the Parsec amplified antenna, that I had on the fireplace mantle would bring them in cleaner. The Parsec could be aimed and that seemed to have an affect. At one time I used an outdoor omni directional antenna (2 folded dipoles stacked one over the other in 90-degree config) leaning up against the wall of the listening room, and there were times when that tuned in the cleanest.
As always seems to be the case, a roof mounted antenna was not an option.
With all of the above said, I had cleaner sounding reception in the Seattle area.
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