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FM folded dipole and coax colinear antenna...

R

radiopilot

I am using a folded dipole antenna of a matched frequency for my FM transmitter and it's great but I was wondering if anyone has ever built the coax colinear antenna using LM-400 coax cable?

The colinear antenna is supposed to give a 3.4-6db gain depending on the number of elements used has anyone has ever built one, what is the experience with this type antenna? I've also seen this antenna in conjunction with a folded dipole antenna, has anyone seen or heard of this?

Radiopilot
 
> I am using a folded dipole antenna of a matched frequency
> for my FM transmitter and it's great but I was wondering if
> anyone has ever built the coax colinear antenna using LM-400
> coax cable?
>
> The colinear antenna is supposed to give a 3.4-6db gain
> depending on the number of elements used has anyone has ever
> built one, what is the experience with this type antenna?
> I've also seen this antenna in conjunction with a folded
> dipole antenna, has anyone seen or heard of this?
>
> Radiopilot
>
Hi,

I built one for 2 meter ham use with RG-8 cable. It had a very good match to the transmitter, but did not have much gain. I din't measure the gain, just campared it to other antennas. Better than a 1/4 wave ground plane but lower than a "ringo ranger 2" which is a colinear with a phasing stub and gamma match (advertised 5 dB or so gain). I seem to recall that I used four coax sections and gamma matching. I gave it away.

Neil
 
> > I am using a folded dipole antenna of a matched frequency
> > for my FM transmitter and it's great but I was wondering
> if
> > anyone has ever built the coax colinear antenna using
> LM-400
> > coax cable?
> >
> > The colinear antenna is supposed to give a 3.4-6db gain
> > depending on the number of elements used has anyone has
> ever
> > built one, what is the experience with this type antenna?
> > I've also seen this antenna in conjunction with a folded
> > dipole antenna, has anyone seen or heard of this?
> >
> > Radiopilot
> >
> Hi,
>
> I built one for 2 meter ham use with RG-8 cable. It had a
> very good match to the transmitter, but did not have much
> gain. I din't measure the gain, just campared it to other
> antennas. Better than a 1/4 wave ground plane but lower
> than a "ringo ranger 2" which is a colinear with a phasing
> stub and gamma match (advertised 5 dB or so gain). I seem
> to recall that I used four coax sections and gamma matching.
> I gave it away.
>
> Neil
>

Neil,

Thanks for the info, the 'ringo ranger' sounds like the one I'm talking about but this antenna uses the LM-400 coax as the dipole itself... see link below:

http://www.qsl.net/sv1bsx/gammadip/collinear2.html

In the above model the coax is RG-59/RG-58 and the gain is 5.15dbi...

Anyway you say you had not as good results with this type antenna on the 2 meter band, never tried it on the FM band?

Radiopilot
 
Sorry, wrong antenna

Hi Radiopilot,

I am glad you sent a link. That is not the antenna that I built. The one I built is in the ARRL Antenna Handbook and consisted of lengths of coax soldered together. At the joints, you solder the shield to center and center to shield to shift the phase. It had a gamma match on the bottom. I was interested in it because it did not require a ground plane, but neither does the Ringo. This design is available as a "Station Master" and has to be factory tuned, and is $$$$.

I was thinking of the wrong antenna. The one on the link looks like a stacked dipole with a phasing harness. I did use a commercial one on a repeater. It was a four bay dipole with a phasing harness and is very similar to the one on the link. This worked very well and allowed setting a pattern by rotating the dipoles around the mast.

The Ringo Ranger was made by Antenna Specialists, but I think they quit making them. Mine has been in service for almost 30 years. Presently, it is mounted on my chimney at a height of 25 feet. I can work simplex about forty miles with it using 25 watts. It is simple in design and adjustment, compact, and probably if one is clever with the insulating materials, one could copy it.

I also suggest you look at the J-pole design. These work well, but in my experience they are tricky to tune properly. Trimming the length for resonance and tapping for proper load interact and it ends up being a back and forth job.

I hope I am not making things complicated for you but I just wanted to tell you that my previously posted comments don't apply to the antenna you are looking at.

Neil
 
Re: Sorry, wrong antenna

> Hi Radiopilot,
>
> I am glad you sent a link. That is not the antenna that I
> built. The one I built is in the ARRL Antenna Handbook and
> consisted of lengths of coax soldered together. At the
> joints, you solder the shield to center and center to shield
> to shift the phase. It had a gamma match on the bottom. I
> was interested in it because it did not require a ground
> plane, but neither does the Ringo. This design is available
> as a "Station Master" and has to be factory tuned, and is
> $$$$.
>
> I was thinking of the wrong antenna. The one on the link
> looks like a stacked dipole with a phasing harness. I did
> use a commercial one on a repeater. It was a four bay
> dipole with a phasing harness and is very similar to the one
> on the link. This worked very well and allowed setting a
> pattern by rotating the dipoles around the mast.
>
> The Ringo Ranger was made by Antenna Specialists, but I
> think they quit making them. Mine has been in service for
> almost 30 years. Presently, it is mounted on my chimney at
> a height of 25 feet. I can work simplex about forty miles
> with it using 25 watts. It is simple in design and
> adjustment, compact, and probably if one is clever with the
> insulating materials, one could copy it.
>
> I also suggest you look at the J-pole design. These work
> well, but in my experience they are tricky to tune properly.
> Trimming the length for resonance and tapping for proper
> load interact and it ends up being a back and forth job.
>
> I hope I am not making things complicated for you but I just
> wanted to tell you that my previously posted comments don't
> apply to the antenna you are looking at.
>
> Neil
>


Neil,

I built the 'j-pole' and the 'slim jim' design and felt the tuning the antenna proved difficult and obtaing a good swr was difficult.

I was hoping with the above design, if I got the proper lengths for my frquency and had good soldered connectons the swr should adjust almost to a reasonable 1:5 or better.

I will build the antdenna in the link above and see if it has the gain as it suggests, then I'll post the results if anyone wants to upgrade their FM antenna... but for the most bang for the buck right now my folded dipole has proven quite good... and inexpensive as it uses 300 ohm twin lead and a matching balun to 50 ohms....

Radiopilot
 
Re: Sorry, wrong antenna

> I will build the antdenna in the link above and see if it
> has the gain as it suggests,

From the website diagram, it should have about the gain they claim -- but that gain is in dBi, not dBd. To get the gain in dBd, subtract 2.15 from their number.

The gain of about 3 dB comes from the fact that there are two vertical dipoles each with a gain of 0 dBd (2.15 dBi) whose fields combine in the horizontal plane.

The radiation pattern will be nearly omni toward the horizon, and be directional in the vertical plane, with peak gain toward the horizon. There should be a pattern null in vertical alignment with the dipoles, so the gain measurement should not be taken there. There are also two nulls in the elevation pattern, with one of them directed toward the ground not too far from the antenna -- so measuring gain there is a problem.
The best place to check the gain will be 500 feet or more away from it, to be sure you are looking at the ~ maximum relative field.

//
 
Folded dipole with balun

Hello again,

I just wanted to add something for others who read these posts. You wrote:

... but for the most bang for the buck right now my folded dipole has proven quite good... and inexpensive as it uses 300 ohm twin lead and a matching balun to 50 ohms....

I put one of these together for my son who has a Ramsey FM25 and just wants to cover his back yard. For those who are interested, this is a cheap and decent antenna. Just use a balun and set it vertically. The only problem is you will probably need to buy 100 feet of twin lead just to use 3 feet of wire. You will also need coax (or put the balun at the transmitter and run twin lead) to connect the Tx to the balun. Maybe you have a friend or someone at work who is a ham, or a tv shop has some scraps of cable on hand.

Neil
 
Question for rfry antenna gain

> From the website diagram, it should have about the gain they
> claim -- but that gain is in dBi, not dBd. To get the gain
> in dBd, subtract 2.15 from their number.
>
> The gain of about 3 dB comes from the fact that there are
> two vertical dipoles each with a gain of 0 dBd (2.15 dBi)
> whose fields combine in the horizontal plane.
>

Hi,

Just a question. I thought the gain of a half wave dipole was 1.5 dB compared to an isotropic radiator. Am I confusing this with an elementary doublet or is this the same as a dipole?

Thanks,

Neil
 
Re: Question for rfry antenna gain

> Just a question. I thought the gain of a half wave dipole
> was 1.5 dB compared to an isotropic radiator. Am I
> confusing this with an elementary doublet or is this the
> same as a dipole?
_______________

Hello, Neil -

The peak directivity of a short doublet is 1.5, or 1.76 dBi gain. The peak directivity of a 1/2-wave dipole is 1.64, or 2.15 dBi gain.

These values can be calculated quickly using NEC, or with more work using standard equations. If interested you might check Table 6-2 in Kraus' "Antennas For All Applications," 3rd Edition, which has a nice summary of field patterns, directivities and gains for quite a few antenna configurations (elemental radiators, and arrays).

BTW, I read your bio here. What college/university were you connected with?

Rich ([email protected])
 
Re: Folded dipole with balun

> Hello again,
>
> I just wanted to add something for others who read these
> posts. You wrote:
>
> ... but for the most bang for the buck right now my folded
> dipole has proven quite good... and inexpensive as it uses
> 300 ohm twin lead and a matching balun to 50 ohms....
>
> I put one of these together for my son who has a Ramsey FM25
> and just wants to cover his back yard. For those who are
> interested, this is a cheap and decent antenna. Just use a
> balun and set it vertically. The only problem is you will
> probably need to buy 100 feet of twin lead just to use 3
> feet of wire. You will also need coax (or put the balun at
> the transmitter and run twin lead) to connect the Tx to the
> balun. Maybe you have a friend or someone at work who is a
> ham, or a tv shop has some scraps of cable on hand.
>
> Neil
>


Actually, I bought a 40 foot roll at RadioShack for $4.00 (most stereos often supply the ribbon dipole) and used only what I needed for the dipole, and for the cable I used LM-400 cable which was given to me by a friend who owns a cable installing service... if you can find the LM-400 cable free or very cheap, it's the choice to use, 75 ohm cable will work also... I'm getting some pretty good distances with my transmitter but won't say... not to start another firestorm.

Radiopilot
 
Re: Sorry, wrong antenna

> > I will build the antdenna in the link above and see if it
> > has the gain as it suggests,
>
> From the website diagram, it should have about the gain they
> claim -- but that gain is in dBi, not dBd. To get the gain
> in dBd, subtract 2.15 from their number.
>
> The gain of about 3 dB comes from the fact that there are
> two vertical dipoles each with a gain of 0 dBd (2.15 dBi)
> whose fields combine in the horizontal plane.
>
> The radiation pattern will be nearly omni toward the
> horizon, and be directional in the vertical plane, with peak
> gain toward the horizon. There should be a pattern null in
> vertical alignment with the dipoles, so the gain measurement
> should not be taken there. There are also two nulls in the
> elevation pattern, with one of them directed toward the
> ground not too far from the antenna -- so measuring gain
> there is a problem.
> The best place to check the gain will be 500 feet or more
> away from it, to be sure you are looking at the ~ maximum
> relative field.
>
> //
>

Rfry,

Thanks... I figured the writer of the article may have corrected this on another page, but did not want to comment about it....

As such I will try to measure the gain. If I get 3db gain, I'll be happy with this... as I only want the signal to be approx. 1/4 mile or less during the skywave hours, during the day my AM transmitter takes over....

Thank you for taking the time to add some input...

Radiopilot
 
Re: Folded dipole with balun

Neil, twin lead is your friend! For long runs the losses are microscopic if it's installed correctly (avoid twists, keep it a few inches away from metal objects). It's well worth the extra trouble for a folded dipole set up 100' or more away from the operating position. -- JasonW

> Hello again,
>
> I just wanted to add something for others who read these
> posts. You wrote:
>
> ... but for the most bang for the buck right now my folded
> dipole has proven quite good... and inexpensive as it uses
> 300 ohm twin lead and a matching balun to 50 ohms....
>
> I put one of these together for my son who has a Ramsey FM25
> and just wants to cover his back yard. For those who are
> interested, this is a cheap and decent antenna. Just use a
> balun and set it vertically. The only problem is you will
> probably need to buy 100 feet of twin lead just to use 3
> feet of wire. You will also need coax (or put the balun at
> the transmitter and run twin lead) to connect the Tx to the
> balun. Maybe you have a friend or someone at work who is a
> ham, or a tv shop has some scraps of cable on hand.
>
> Neil
>
 
Neil, which 50 ohm/300 ohm balun?

Hello Neil,

Which 50 ohm/300 ohm balun did you use for your son's Ramsey FM25?

I've been thinking about setting up a low-mounted FM folded dipole just to cover my property.

I know that an ordinary 75 ohm/300 ohm TV balun would also work at Part 15 FM power levels, and its 1.5:1 SWR mismatch at the input would act as an attenuator to keep the radiated power low (a good thing if your intention is only to cover your property). -- JasonW

> Hello again,
>
> I just wanted to add something for others who read these
> posts. You wrote:
>
> ... but for the most bang for the buck right now my folded
> dipole has proven quite good... and inexpensive as it uses
> 300 ohm twin lead and a matching balun to 50 ohms....
>
> I put one of these together for my son who has a Ramsey FM25
> and just wants to cover his back yard. For those who are
> interested, this is a cheap and decent antenna. Just use a
> balun and set it vertically. The only problem is you will
> probably need to buy 100 feet of twin lead just to use 3
> feet of wire. You will also need coax (or put the balun at
> the transmitter and run twin lead) to connect the Tx to the
> balun. Maybe you have a friend or someone at work who is a
> ham, or a tv shop has some scraps of cable on hand.
>
> Neil
>
 
Re: Neil, which 50 ohm/300 ohm balun?

> Hello Neil,
>
> Which 50 ohm/300 ohm balun did you use for your son's Ramsey
> FM25?
>

Oops! This one wasn't a typo. I was in a hurry because I had an appointment and was going to be late. That's why I usually post late at night.

It was a 4:1 balun, so that would make it 75 ohms, it was a standard TV type balun. I used 50 ohm coax to it since I had some at hand and I figured a 1.5 to 1 SWR wouldn't make much difference.

Reference your previous comment about twin lead for a feed line, yes it is low loss.

Neil
 
Re: Sorry, wrong antenna

>... I only want the signal to be approx. 1/4 mile
>or less during the skywave hours, during
> the day my AM transmitter takes over....
_______________

Even a simple 1/2-wave dipole with 0 dBd gain takes very little input power to generate a receivable signal 1/4-mile away over a clear, reflection-free path.

Below are two examples. The field strength values are based on the inverse field equation.

Power Radiated by Antenna > Field at 3 meters > Field at 402 meters (1/4 mile)

11.5 nanowatts > 250 uV/m > 1.87 uV/m
33 microwatts > 13,420 uV/m > 100 uV/m

Doubling those powers at the tx output likely would more than cover any losses in the antenna feedline and balun (if used) -- and still the tx output power needed will be very small.

So unless you just want to experiment with building the collinear, for your goal here probably you don't really need the extra gain it might have.

//
 
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