Building dedicated Antenna

[Rainer Ehlert]

Hi,

I have read a lot about building an Antenna but one think is still not clear for me and that is the famous Balun.

So, how do I know when I need one and how do I know what type of Balun I have to use.

My actual antenna has a so called impedance transformer on it and then I use a 75 Ohm cable and with an F Coax cable adapter to SMA I connect the antenna to the SDR.

I found a calculator for a 4 element Yagi for 174.31 MHz and now I am totally lost about the famous Balun.

Thanks for any tips

Rainer

[Dennis Condron]

The balun transformer of the typical type used on TV antennas allows one to use it to convert a higher impedance such as 200-300 ohm range down to a 75 ohm coaxial line now used by almost everyone. it is by definition a 4:1 impedance transformer

It serves two purposes  - to transform the higher impedance to a lower one and also is performing a balanced antenna input to an unbalanced load (the receiver)

A Yagi antenna can be used without a balancing transformer by using what is called a Gamma match on the Yagi driven element. An example is included in these links : http://www.dx-antennas.com/Gamma_match.htm - http://www.arrl.org/searches/results

Hope this helps you rainer !

73 - Denny - K0LGI

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[Rainer Ehlert]

Hi Denny,

Thanks. Is there a less complicated solution ? :-)

The Yagi calculator I used has this line

A half wave 4:1 balun uses 0.66 velocity factor RG-216 (PE) and is 568 mm long plus leads

So I guess that I can put on the dipole and then connect the normal F connector for my 75 Ohm coaxial line downwards. Correct ?

Can I use 2.5 mm copper wire ? for the reflector, dipole and directors ? If not then I would need to get some 3/8" or 1/2" Aluminum tube.

Attached an image of the support for the refelctor, dipole and director. Please if that is nonsense please tell me. Here in Mexico it is difficult to get material as you are used in USA:

Thanks and regards Rainer

On Friday, September 25, 2020 at 10:36:39 AM UTC-5, Denny - K0LGI - wrote:

The balun transformer of the typical type used on TV antennas allows one to use it to convert a higher impedance such as 200-300 ohm range down to a 75 ohm coaxial line now used by almost everyone. it is by definition a 4:1 impedance transformer

It serves two purposes  - to transform the higher impedance to a lower one and also is performing a balanced antenna input to an unbalanced load (the receiver)

A Yagi antenna can be used without a balancing transformer by using what is called a Gamma match on the Yagi driven element. An example is included in these links : http://www.dx-antennas.com/Gamma_match.htm - http://www.arrl.org/searches/results

Hope this helps you Rainer !

[Dennis Condron]

I did not receive your attachment of the Yagi design you are describing Rainer. 

For now, I would build up the Yagi design you have in mind first, using practically any conductor you have that is reasonably stiff enough to support its own weight. For 174 MHz that should not be an issue when using solid wire or tubing, then I would suggest using the Gamma matching mainly because of its simplicity.

The Gamma matching section shown in pictorial form, is in reality an adjustable capacitor made using the tubing as one side of the capacitor which by sliding it in or out of the tube determines the capacitor value need to cancel the driven element (inductive) Xl value it created to match the antenna driven element to the feedline impedance. 

It can be substituted with a real capacitor such as an air variable, or ceramic capacitor of a few 10's of pF if it is enclosed for weatherproofing. The Gamma matching capacitor purpose is to negate the Xl created by the matching section that connects the feedline on one side further out on the driven element.

If nothing else Rainer you can connect the coax cable directly to the Yagi driven element for initial testing, however performance will be better when the coax used is matched to the antenna impedance using the above Gamma matching method. There are other more sophisticated methods of impedance matching as well that can be used also, but usually require more material and adjustments required to implement.

Denny

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[Rainer Ehlert]

Hi Denny,

Thanks a lot :-) for those tips and I will work more and more in order to optimize all this.

Well I think I am one step ahead :-) The antenna is finished and it is connected to my SDR.

First made a screenshot with the commercial Multifrequency antenna. You can see that in the image named

Volteck-ANDO-4-Yagi_Multiband

Then I connected my DIY 4 element Yagi and the signal is like that with no LNA connected.

DIY_4-Element-Yagi-174.31-MHz

Then I added the LNA I have bought from RTL and the signal looks clean again and as strong as the commercial Yagi.

DIY_4-Element-Yagi-174.31-MHz_with_LNA_connected

From the antenna I have about 10 inches of 75 Ohm RG-6 coaxial cable , then the LNA and then 15 feet coaxial cable to the SDR. The Bias T is switched on in the RTL-SDR v3.

I think that it is good for being my first antenna :-) and I just saw I am getting scatter on the screen :-). Will let it run overnight inside my room. I need to make a new support for getting it outside and think how I can waterproof the LNA.

Attached also an image of it. The copper wire is a bit flimsy but is what I could get for a start. Looking at it now I can go and buy Aluminum tube with 3/8" diameter and make a new one.

I find this really interesting especially DIY and then see that it works :-)

regards Rainer

[Rainer Ehlert]

Hi,

... and the Antenna is outside and the signal is a bit stronger, -30 dB ? and I see it a bit cleaner

:-)

Rainer

[Dennis Condron]

Looking good Rainer !

It is an interesting construction method that looks very strong too. 

Is there a link to where the design is discussed ?

Good to see it is working well for you !

Denny

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<4-Yagi_174MHz_Antenna_Outside.jpg>

<4-Yagi_174MHz_Antenna_Outside_cleaner.jpg>

[Rainer Ehlert]

Hi Denny,

Thanks. Do you mean the design of the PVC pipes ? As I do not find PVC crosses here I deducted this from a MOXON holder :-) but made it as narrow as possible. Perhaps I should cut down the " T " for the mast and so I get it narrower.

Will see what I got tomorrow morning.

Rainer

On Friday, September 25, 2020 at 7:49:00 PM UTC-5, Denny - K0LGI - wrote:

Looking good Rainer !

It is an interesting construction method that looks very strong too. 

Is there a link to where the design is discussed ?

Good to see it is working well for you !

Denny

On Sep 25, 2020, at 7:02 PM, Rainer Ehlert <rsf...@rsfotografia.com> wrote:

Hi,

... and the Antenna is outside and the signal is a bit stronger, -30 dB ? and I see it a bit cleaner

:-)

Rainer

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<4-Yagi_174MHz_Antenna_Outside.jpg>

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[Dennis Condron]

I think if it were me I would leave PVC spacing as it is now Rainer. It would be a stronger support frame I believe that way.

I was asking if you had a website link for the antenna design details, mechanical and electrical.

It is good that you have it mounted outside now, as in the house generated spurious noise sources are usually less that way.

Shutting down for the evening on this end. Good luck tomorrow morning with your new antenna !

Denny

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[Rainer Ehlert]

Hi Denny,

Well it worked for 11 hours and after checking not one single scatter ...

Is that possible or did something go wrong ?

For calculating the Yagi I used this calculator   https://www.vk5dj.com/yagi.html

and the PVC frame slowly developed in my head :-) basing my idea on a Moxon frame but added a second T-joint for the near director to the dipole. On the dipole a third T-joint in the center allows me to tilt it. The whole frame is a bit heavy. Perhaps the enxt one I will drill holes into the pipe to get less wight and perhaps make it less susceptible to wind.

For the distances of the reflector, dipole and directors I took as reference the center of the 90° elbows and the center of the " T " joint. In this case it is 1/2" pipe.

regards Rainer

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[Dennis Condron]

Sorry to hear about the lack of receiving signals Rainer.The morning hours are typically the best time for meteor returns to be received.

Are you sure something did not loosen when puting the antenna outside ? Does the LNA unit have power and is enabled by the SDR unit ? Also verify if the coax connections have not come off or are loose fitting are some of the things to check. 

Something must have changed after it was moved outside if it was working OK inside the house before the move.

Denny

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[Rainer Ehlert]

Hi Denny,

Will check everything. I am sure nothing is loose but anyhow I will check it again. :-(

Normally I should have got some at 174 308 440 Hz as from my point of view it does not matter which antenna I ahve connected ...

Yes the LNA has power from the SDR.

Well, nothing is perfect :-) or perhaps Mr. Murphy is here :-)

Rainer

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[Dennis Condron]

Murphy is a common visitor here as well Rainer ! I believe he must live very close as the Murphy effects often come out of nowhere :)

It seems Slavador, yourself, and me are doing antenna projects all at about the same time. Most of my antennas were missing or destroyed in a derecho wind event that occured in early August that is still an ongoing project to build what I can back to normal again.

Hang in there !

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[Rainer Ehlert]

Hi Denny,

I found the problem.

If you look at the image of the antenna outside you will see two green wires wound aroung the PVC mast and they were connected to the steel base at the bottom. This I used for grounding the commercial antenna. Now they were just there and doing nothing and I guess they were acting like an antenna dping some strange things. When I took them off immediately my signal was cleaner.

Now without LNA powered ON I get a signal of -50dB. Powering on the the LNA via Bias Tee I get a signal of -35dB to -30dB. I think that is quite an improvement. The signal you see at 174 308 200 has been always there with the commercial antenna as well as with the 4-Yagi in my office.

Radio waves are really mysterious stuff.

Rainer

[Rainer Ehlert]

Hi Denny,

Everything ok again. On the attached a daytime scatter.

Rainer