I purchased the fm omini directional antenna at the link below for a low powered transmitter.
Does the lower loop on this antenna act as a ground plane, or do a need to add a circular piece of metal below it for a ground plane?
http://www.ebay.com/itm/251802680044?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Thanks

 

No.....that's called a folded dipole. The elements work against each other....not the ground. And you shouldn't need a ground plane. The distance to ground potential can effect radiation angle.

There is a balun/small transformer and possibly some caps behind the connector. So depending on what you call low power......be aware of this. And if you turn the antenna vertical.......you might get some directionality from it.

I wouldn't use a bent antenna for transmitting. It's bad juju.

 

No.....that's called a folded dipole. The elements work against each other....not the ground. And you shouldn't need a ground plane. The distance to ground potential can effect radiation angle.

There is a balun/small transformer and possibly some caps behind the connector. So depending on what you call low power......be aware of this. And if you turn the antenna vertical.......you might get some directionality from it.

I wouldn't use a bent antenna for transmitting. It's bad juju.

Thanks for the info. I am an experienced electronics technician but have no rf or antenna knowledge. This is for a 1W transmitter. The internet says a folded dipole is usually around 200-300ohms but the specs for this antenna say it is 50ohms; which is what I needed (the transmitter is also 50ohms). Does that mean the cap & balun are used to adjust/match the impedance?

 

Yes...a 300/75 ohm transformer. Might not be any caps. If you use that antenna...be sure to check...that unit is built as a receiving antenna.

Not sure what you're doing with 1 watt transmitter.......but I would use 1/4 to 5/8 ground plane vertical. For less noise and better rad pattern. A mountain top really helps. I have put private 2-5 watt repeaters, out west....and got hundreds of square miles coverage.

 

No.....that's called a folded dipole. The elements work against each other....not the ground. And you shouldn't need a ground plane. The distance to ground potential can effect radiation angle.

There is a balun/small transformer and possibly some caps behind the connector. So depending on what you call low power......be aware of this. And if you turn the antenna vertical.......you might get some directionality from it.

I wouldn't use a bent antenna for transmitting. It's bad juju.

I would connect the transmitter directly to the antenna and not worry about needing a balun.

 

I would connect the transmitter directly to the antenna and not worry about needing a balun.

Thanks for the suggestion. The antenna apparently has the equivalent of a balun already built in (black connection box). Since it is rated at 50 ohms, it seems it will do what needs to be done. I have no antenna experience but I have seen folded dipoles in books... it just never occurred to me that bending a dipole circular would give omini directional characteristics - which I need.

I had considered making a broad band "cone" antenna but I would be in over my head. As the other individual noted, it is a receiving antenna but any good engineering book will remind us, "a good receiving antenna makes a good transmitter". There was no instruction sheet other than the radiation pattern which is nearly omni directional. Unfortunately, the pattern does not show the orientation to the the physical antenna design. The patter shows two small spikes 180deg apart - I assume this must be the antenna ends.

 

That antenna has a 300 ohm impedance. You can use it as a transmitting antenna, but it would require a BALUN. I can't tell from your picture, but there doesn't seem to be one on your antenna. If there IS one, it would not be good for transmitting. The cone antenna is by far a better antenna for transmitting. You don't say what polarization that you need, but your picture is a horizontal polarized antenna. A cone antenna is no more than a vertical transmitting element, with either a solid conical or a series of downward pointing radials (6 to 8) that approximate a cone that act as a ground plane. It is quite easy to build and implement and you don't need a balun.

You are obviously over your head with this. Perhaps if you told us the "whats" and "whys" we could better help you.

 

"Perhaps if you told us the "whats" and "whys" we could better help you."

Well... You are mostly right.

I double checked the spec sheet again and the antenna is 75ohm not 50ohm like I thought. I have a tall unused Dish.net antenna pole rising above my roof I plan to use. I am going to use it as a music fm transmitter for my property. I live in rural Montana and there are no radio stations. So, I am setting up a radio that reaches my home, shop, barn and pasture. I only need to broadcast reliably 1/4mi so I was not interested in an efficient antenna. I also did not want something that I had to run guy wires, etc.

So, I guess all I need now is something to match the 50ohm transmitter to the 75ohm antenna. It looks like the transmitter uses an "n" connector and the antenna is a threaded coax?

Antenna Specs
Beam: 360deg
Diameter: 450mm
Gain: 4db
elements:1
Impedance: 75ohm
Freq: 85-110mhz

 

Hello,

The antenna seems to use an F connector.
From the given page:
Frequency range: 87.5 ~ 108Mhz •Maximum gain: 4dB •Impedance: 75ohm, female "F" connector •Diameter: 17.75"
F connectors are similar to the connectors used on sattelite recievers.

Bertus

 

So, I bought this "N" male to "F" 50-75ohm converter and have it on it's way. The transmitter manual does not say what type connector it uses for rf output, but from what I can tell it is a "N". (The antenna connector looks just like the back of a TV - I hope that is "F")

UPDATE: after reciving the impediance converter, it did not fit. The xmit specs said it had a BNC connector but I knew it did not. Come to find out it has a TNC which can look like a "N" connector in photos. So, I had to order an additional adapter for $5.

This is what I know have...
xmitter -- 50ohm TNC femal
adapter -- male TNC to female "N"
impedance -- male 50ohm "N to 75ohm "F"
antenna wire -- 75ohm "F" to 75ohm "F"
antenna -- 75ohm "F"

Whew!!!!

http://www.ebay.com/itm/Uniden-UNI-...&s=ci&page=main:email&cal=1f85abb840ae5&cust=

 

Hello,

It states that the antenna has a female F connector.
You will need a male connector to attach the cable to it.
This can be a connector that can be pressed on the cable:
http://www.ebay.com/itm/20Pcs-CCTV-...831053?hash=item281ceaa30d:g:LUEAAOSwIgNXllfa
A male F to SMA converter:
http://www.ebay.com/itm/F-Type-Male...101169?hash=item4405253731:g:TfAAAOSwq~tZOSJG
Or male F to the more normal TV connectors:
http://www.ebay.com/itm/F-Male-Plug...809398?hash=item4d225a0fb6:g:GF8AAOSwbqpT6yPX

Bertus

 

Thanks!
Now that I think about it, I have some of those connectors left over from installing an antenna on my motor home a couple years ago.

 

At that power level you may get away with it. But I've used the J Pole antenna for many years..
http://www.hamuniverse.com/jpole.html
They are easy to make and rugged too.

 

Hello,

@dendad , I have used a J-pole antenna a long time for my 2 meter rig.

Bertus

 

What can you say about clandestine FM stations in Montana. I have my own little station using a zfmsm connected to a simple coaxial dipole antenna on the side of the house. The output power was turned down to keep coverage just to the lot and to make it ~FCC legal. YMMV

Under Part 15 rules in the U.S., The FCC limits unlicensed FM broadcasts to a maximum signal strength of 250uV/m (microvolts per meter), measured at a distance of 3m--about 10 feet. You also must not transmit in such a way that your signal interferes with licensed radio transmissions. From a practical standpoint, this maximum signal strength will give you a range of about 100 to 300 feet, depending on terrain, obstructions, and receiver sensitivity. With a car radio, you might be able to receive a decent signal up to a few blocks. This should be adequate for covering your house or a small apartment complex.

http://www.nsw.wicen.org.au/technical/projects/coaxial-folded-dipole-antenna

 

Hello,

@dendad , I have used a J-pole antenna a long time for my 2 meter rig.

Bertus

Yes, that is what I have. Also, I've made a 6M one too, but my 6M box died so I've not used it. What I've seen done is a 2M J Pole on the top section of a 6M one. They are a very good antenna and I'd recommend building one.
One day I'll get back to fixing the 6M box!

 

Now, I have turned my attention to programming software. I looked but could not find anything. I want it to automatically randomly choose mp3 songs in genre folders according to time of day (24/7). I also have a unused 450w solar cell array I thought about making it completely autonomous.

8am-5pm -- 70-80-90's medium rock-n-roll
5pm-9pm -- harder rock-n-roll
9pm-8am -- new age, ambient

I already have a miniature linux single board board computer similar to the RPI that I hope to use. I expect to have to write a bash script of some sort. If anybody has any suggestions for cmd line apps that I may want to investigate, I am interested. (I do have a spare win7 laptop I could also use.

 

I use the MPD daemon for my station and icecast2 for online music playing. You should be able to install the software from the repo of the Linux system you are running.

https://www.musicpd.org/

 

https://www.musicpd.org/

YES!
This is what I need to get started

I have not looked at all options but MPC => MPD will at least get me started. I can't wait to start programming!

What I am eventually hoping for is something that will allow me to periodically execute an external script so that I can insert the time of day approximately every 1/2 hour. I have already solved the text to mp3 solution for this.

I can imagine a work around. MPC will allow me to update playlists via cmd line on the fly and I could use a data base to inject a time of day mp3 at the appropriate time - but I would like to eventual come up with a better solution.

I looked at a few of the GUI solutions (Amerek, etc) and none offered what I needed.

 

https://www.musicpd.org/

YES!
This is what I need to get started

I have not looked at all options but MPC => MPD will at least get me started. I can't wait to start programming!

What I am eventually hoping for is something that will allow me to periodically execute an external script so that I can insert the time of day approximately every 1/2 hour. I have already solved the text to mp3 solution for this.

I can imagine a work around. MPC will allow me to update playlists via cmd line on the fly and I could use a data base to inject a time of day mp3 at the appropriate time - but I would like to eventual come up with a better solution.

I looked at a few of the GUI solutions (Amerek, etc) and none offered what I needed.

It would be easy to write a script or program and have cron execute that every half hour.
http://kvz.io/blog/2007/07/29/schedule-tasks-on-linux-using-crontab/