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peace brainerd 4th October 2012 11:12 PM

help me find an FM 1/4 ground plane or 1/2 dipole diy antenna plan for broadcast??
Kind of an unusual application. I won't be feeding it with any length of coax. I'm using a little USB transmitter and connecting the board antenna connection directly to the antenna element.

So I don't know how to adapt it to the antenna plans I've found on the net since they all go into details about balun considerations which I believe don't apply here. Or do they? Like to shoot for a half wave dipole of ultra simple wire construction that will be in an upstairs closet above the workshop where my HP mini does internet radio broadcast duty all day long. Then I can listen to it on any old fm handy out in the garage, in the garden, or next to the firepit.

Transmitter works beyond my expectations ranging approx 50 ft. I'd like to extend that to the garage and back yard. I'd rather go a bit more gain on the antenna and rein it in using the dx sliders in the GUI.

Can anyone advise? I can get a 12 or 15ft usb extension for a couple of bucks. That will be the antenna "lead" rather than any length of coax.

I'm comfortably aware of FCC limits and have read quite a bit on the subject this week. There are Thousands of these "whole house" transmitters in use that are employing higher output than this little "dongle". I do realize that the antenna gain amplifies that, but I"m starting out so small that I'm not concerned. There are also many hundreds of Christmas display hobbiest that are broadcasting a couple of blocks with FM xmitters and antennas many times the size I'm contemplating and any of them have yet to run afoul of the Feds. I'll take great care to use an empty space and keep it within my four acres in the rural outskirts of town.

asmith 4th October 2012 11:36 PM

Just make a 1/2 wave dipole for the freq you are set for and hook the coax to it. With the power your making you dont need to be concerened with matching.

gootee 5th October 2012 12:14 AM

"Antenna Gain" does not increase the radiated power.

Antenna Gain is realized by making the antenna more directional. You get more gain in some directions and less in others. Antenna Gain typically must have a direction associated with it.

An "isotropic" or truly-omnidirectional antenna (which doesn't exist in reality) spreads the power out so that it would fall evenly onto the inside surface of a sphere that was around it (and centered at the antenna's "phase center"). The gain of all other types of antennas is often expressed in dBi, i.e. "dB compared to isotropic", at each elevation and azimuth angle.

If you set up an antenna, such as a vertical dipole, which concentrates much of the radiated power closer to the horizon, with much less being radiated at large angles away from horizontal, then you have increased the antenna gain in the horizontal directions.

A USB extension will most likely NOT work. If you want an extension, use COAX. Why do you think it's used? It's because it doesn't WORK, otherwise. It's like saying, "Well, I don't have any WIRE for my speakers so I'll just use this ROPE I have handy.". What's the reason for not wanting to use coax? You probably wouldn't need actual connectors. You could try just soldering the center wire and the shield to the board and then to the dipole wires at the other end.

You might be able to get away with connecting the antenna directly to the board. Just get two pieces of wire of the proper length and connect one to each antenna terminal, and immediately take one vertically upward and the other one vertically downward.

Wavelength (in METERS) is 300 divided by frequency in MHz. Each wire should be about 1/4 wavelength, I think. But the speed of the waves in the wire is somewhat slower than in air so the "electrical length" of the wires won't be the same as the length calculated using the 300/MHz/4, since that uses the speed in free space. You could try up to 50% longer and cut them down by equal lengths at a time until it gets better and then starts to get worse. I forget the typical rule. Maybe assume the speed (300) is 85% of 300, in the wire, and calculate with that.



geraldfryjr 5th October 2012 01:39 AM

Here is some stuff.

5/8th wave mobile antenna vs 1/4 wave

Amateur Quarter Wave Ground Plane Antenna Calculator

I used to to experiment with with 3/4 wave verticals in the 11meter band and I have had very good results from them especially for local signals.
When matching to a unbalanced output you need to stick with odd multiples of the 1/4 wave length.
I am no expert but what I did learn was that the 3/4 wave produces a lower angle of attack of directivity compared to that of a 1/4 or 1/2 wave vertical, As well as has more gain I believe, also being on odd of 1/4 wavelengths this helps to keep the impedance nearly the same as a single 1/4 wave vertical so matching is easy, as multiples of 1/2 wavelength verticals the impedance is much higher.
It is like hooking many speakers for more gain only they must be match for the frequency involved.
I did go into the 1 1/4 vertical abit but the antenna structure became a big issue especially in the big winds that Florida gets in the late summertime and fall.
I had a most awesome performing 3/4 wave vertical I had was on just the surface of my roof with a 1/4 wave ground plane made of wires.

But as I had said of the structural reasons, I just hung it out of a tree about an 1/8 wave above the ground and end fed it with the center of the coax and as long as the ground was wet it worked great.
I also adjusted it to a 5/8 wave and adsimilar results with it.
It was hard to figure out as conditions with the ground changed from day to day and it worked best in the rain probably due to the ground issues of the soil.
I did try to gamma match it as well, from a piece of the driving element from a Jo-gunn beam antenna that I had got.
This would have made it a J-pole but this was before I knew what I was doing and water would get into the gamma match and short it out rendering me with no signal no matter how much power I used.
But it worked great when it was dry.
I was well known to have one of the most sensitive ears in the area and I never bought a commercial antenna.
The Antron was the thing at the time to beat and at the time they claimed it to be a highest gain antenna.
I do not believe this to this day.

It has been a long time since those days but most of the data I find on the net shows the results I have experienced with my old setup.

But the number one thing is if the impedance matching of the output of the transmitter to the antenna is not as close to 1:1 as you can get you will be wasting a lot of precious range when it comes to the sub 1 watt or sub 100 milli watt range.

I hope this helps !

jer :)

P.S. Sadly I never got my HAM license yet !! He,he,he

sreten 5th October 2012 01:45 AM


Not a radio expert but I know the basics. The USB lead has nothing to
do with radio performance and related comments regarding coax
simply don't apply, the original post is confusing in this regard,
describing the USB cable as the antenna lead, it simply isn't.

Just as you can make an aerial more directive and increase its gain
(compared to a simple dipole) you can make a transmitter more
directional and increase its range in one direction to the other.

Which you would say in the case your a small town station. You'd
probably have a omni transmitter for your town and additional
directional antenna's pointed towards your neighbouring towns.
If they were all 50 ohm and you had 4 you'd clearly wire them
in series/parallel for a 50 ohm coaxial feed from the amplifier.

(This may be simplistic, one complex array can do all the above.)

Polarisation (vertical or horizontal or mixed) is also an issue.

I'm not an expert but aerials as receivers and transmitters are
entirely interchangeable, and you can build a transmitting array
to suit your purpose. Baluns and the like are a more efficient
way of getting impedance matching, and/or power distribution,
resistors a more basic method.

Perhaps the most useful comment I can make is comparing
vertical and horizontal single dipoles as receivers or transmitters.

Vertical is far superior as a short range ground transmitter as
it has the same range as horizontal without the latters dipole
directionality - which makes that far better to air in the two
directions of the dipole. Ground to ground the dipole aspects
of a vertical don't really come into any considerations.

So assuming vertical reception, if your reception area is circular
simply use a single vertical dipole transmitter. However if its more
oval an aligned two element vertical transmitter may be more suitable.
The more extreme the oval shape, the more aerial elements you use.
(I said aerial elements because i'm assuming an aerial is the transmitter.)

rgds, sreten.

peace brainerd 5th October 2012 04:53 AM

There should be no confusion at all if you read my post closely and opened the pictures linked. I never referred to the USB cable as any part of the antenna element. I did say that the circuit board on the USB "dongle" will be connected DIRECTLY to the base of the antenna elements, effectively negating any need for a length of coax between the antenna and the transmitter (as is usually the case and why the confusion here.)


I'm using a little USB transmitter and connecting the board antenna-connection directly to the antenna element.
I was hoping that made it sufficiently obvious that I was not substituting COAX cable for USB. I should have mentioned that the USB device would be permanently mounted to the plastic juction between the rabbit ear elements.

So, the 10foot USB cable connects the PC to what is now a "smart" antenna. Usb cable is not an antenna signal cable in any way. The transmitter itself is a little 1/2" by 2" USB circuit board permanently attached to the union between the two rabbit ear elements.

So my main question is, now that coax has been eliminated entirely is balancing any concern at all beyond getting the two elements at the same length?

madtecchy 5th October 2012 05:22 AM

It is still important for good signal transmission to match the impedance of the antennae with that of the transmitter... In your case you have very little power so good matching is very important as Goote says.. with a dipole type antennae you can alter the impedance by altering the angle of the ground plane relative to the radiator. Hey this is bringing back memorys of the inverted v antennae on 11meters.. .. I have used long usb leads to feed a wifi dongle that was mounted directly on a 100cm satelite dish.. This was at 2.4Ghz so getting rid of the coax was a real benefit. The main problem with extending usb is with voltage drop on the 5v line .. Have managed 50 meters using powered hubs with a hub every ten meters . Did have to run a dc cable parrallel to the usb cable at 12 v then a 5 v regulator at each hub ..

peace brainerd 5th October 2012 06:18 AM

I've only run wifi and printers and such on lengths up to but not exceeding 15 feet without any problem in voltage loss or signal (packet?) confusion.

I'm confident that a 12ft USB will work just fine with this.

A couple of pics might clear it up for those who haven't gotten their head around the 'no coax needed' mounting of the USB extended xmitter directly on the junction body of the rabbit ears.

Very rough but just to get the gist of it. The card solders right to the stubs of the copper flat lead coming off of the antenna base. A PVC tee can be assembled around the finished job for protection even though this is staying in a closet upstairs.

DF96 5th October 2012 11:06 AM

If you want maximum range then you will need to find what impedance the transmitter expects to see (could be 35, 50 or 75 ohms), and whether it is balanced or unbalanced (probably unbalanced). Then you need an antenna, possibly impedance matching and possibly a balun. Note that coax does not introduce imbalance; it is the (probably) unbalanced transmitter which does that.

On the other hand, if you are not too bothered then put the device at the centre of a half-wave dipole (total length about 95% of free-space half-wave) and accept a possible mismatch and some radiation from the USB cable instead of the antenna. In most cases this will be adequate.

peace brainerd 6th October 2012 03:32 PM

What, in layman's terms, is the difference between a 1/4 wave ground plane

... and a 1/2 wave dipole?

The semantics throw me a bit. They are described everywhere as completely different animals.

The first is a mast element fed by the signal side, provided with a ground plane of elements connected to the ground side.

The second is a mast element fed by the signal side, provided with a ground plane of a single element connected to the ground side.

What makes them so fundamentally different beyond the fact that the main element is tuned for 1/4 or 1/2 wave?

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