Simple dipole for FM-BC reception. To balun or not to balun?

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Kenn
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Joined: Fri Apr 08, 2016 8:43 pm

Simple dipole for FM-BC reception. To balun or not to balun?

Post by Kenn » Tue Feb 07, 2017 5:31 am

Greetings! I'm working on a simple dipole for FM band (88-108MHz) reception. Two 30" segments of wire fed with RG6 in the middle seems to be working well, especially when placed on the roof. In interest of optimizing the antenna, I'm looking into baluns.

It's my understanding that a simple dipole presents ~75 ohms at the feed-point so no impedance matching is required.

However, some sources indicate that without a 1:1 balun, the feed-line might become part of the antenna system which may degrade reception by picking up stray RF or multipath along the feed-line. Others say a balun is not needed for a simple dipole.


So I ran three experiments:

First get a baseline signal strength of all stations receivable at my location using GQRX (AGC off) and a plain dipole, no baluns. (Note there's no perceptible interference issues at this location).

Second, install a 1" long ferrite choke around the coax at the feed-point. Two stations improved a bit, ten went down, the rest stayed the same.

Third, remove the ferrite choke and neatly coil 1/2 wavelength of coax three time at the feed-point. Again, a net decrease in performance.


Everything I thought I knew previously indicates coils of coax are to be avoided. Perhaps in my situation the feed-line is part of the antenna and is constructively adding to the signal? Perhaps a different balun design? Or that a baluns are really only relevant when transmitting?


What do you think?

-Kenn

snn47
Posts: 215
Joined: Tue Dec 27, 2016 11:00 pm

Re: Simple dipole for FM-BC reception. To balun or not to ba

Post by snn47 » Tue Feb 07, 2017 8:04 am

Most FM-BC antennas are vertically polarized, unless you want directivity, a 1/4 wave ground plane is mechanically shorter. A 1/4 wave ground plane is basically a 1/2 wave dipole, were the 1/4 vertical radiatior is reflected by a conducting ground/counterpoise or 1/4 wave radials, it is asymetrical and therefore no need for a BAL(enced)UN(balenced). You can hang a dipole upside down to achieve vertical polarization to achieve vertical polarization.

Depending on where you live FM-BC stations may in part also transmitt some of their power horizontally, but that is not the norm.

Most car whip/telescopic antenna used to be for this reason "vertically" mounted, but were short in relation to the wavelength and only matched for a few MHz {wavelength [m] = 300 000/f [MHz]}. Since FM-BC-stations transmitter can be as high as 100 000 Watt for the stronger ones a short whip workes as everyone knows. Todays car antennas are just very short compared to the wavelength but compensate this suffently by using a preamplifier behind it.

Mechanically such a car antenna could be a solution for you as well.

Do you still want to know about the dipole?

rtlsdrblog
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Re: Simple dipole for FM-BC reception. To balun or not to ba

Post by rtlsdrblog » Tue Feb 07, 2017 8:29 am

If the feedline is acting as part of the antenna, it can skew the radiation pattern. So it could be skewing it towards higher gain in the direction of the FM transmitter, and weakening it in other directions. Adding the choke prevents the feedline from skewing the pattern, which may be worse in your situation.

Kenn
Posts: 51
Joined: Fri Apr 08, 2016 8:43 pm

Re: Simple dipole for FM-BC reception. To balun or not to ba

Post by Kenn » Wed Feb 08, 2017 7:34 am

Thanks snn47 & rtl for your feedback!

I'm located in Arizona, US. Most stations here are using both horizontal and vertical polarization. I too like the simplicity of a 1/4w vertical, but it seems horizontal works a bit better at my QTH. It's position sensitive, perhaps due to the slight directionality of a dipole.

I made a folded dipole (300 ohms) with a matching transformer at the feedpoint for use with with RG6 (75 ohms). It pulls in stations 100 miles away, one is even strong enough for HD mode.

Is the folded design better or might the right balun bring the simple dipole up to par? I'd like to make a few for family & friends and the simple dipole is a little easier to build. Otherwise I can stick with what works.


-Ken

snn47
Posts: 215
Joined: Tue Dec 27, 2016 11:00 pm

Re: Simple dipole for FM-BC reception. To balun or not to ba

Post by snn47 » Wed Feb 08, 2017 11:46 am

A dipole should give you a =8= antenna pattern to dipole elements shown as ==, meaning if you rotate it by 90° horizontally the signal of a station which you had at max. before, should decrease significantly or not be receiveable at all. If not it does not work as a folded full wave dipole.

There were resonably priced logarithmic antenna (LP-antenna) available that started ~ at the FM-BC band and covered the VHF to most UHF TV-channel. A LP is similar wide in bandwidth to a Discone, but can provide Gain in excess to ~6 dBi, depending on design and bandwidth choosen, but unlike a Bi-directional Dipole it is directional.

For polarization, unless you have unobstructed Radio-Line of sight there are a lot of propagation effects that are involved that can give you better reception at one time with vertical at another time with horizontal polarization. HAM operator therefore offen have a crossed dipole, where they can select vertical, horizontal or a combinations of both.

Kenn
Posts: 51
Joined: Fri Apr 08, 2016 8:43 pm

Re: Simple dipole for FM-BC reception. To balun or not to ba

Post by Kenn » Thu Feb 09, 2017 8:33 am

Roger that. I plan to experiment with some directional antennas soon. I'm amazed at what I can get at my location. With the exception of two low power translators on a mountain ~6 miles away, everything else is 27 to 100 miles away, either behind behind one or two mountains or well over the visual horizon. (Radio horizon is farther). Knife edge effect. So yes, propagation effects are felt here. Some days things boom in loud and clear, other days it's pretty weak. It complicates comparison testing, because results are so different from day to day.

Would you think a Cubicle Quad might be responsive to various types of polarization?

Also of possible interest to some, I've found two good sites for determining what might be available at a given location and it confirms I'm in a bit of a sweet spot, given a modest antenna installation. The heat maps are a lot of fun! Unfortunately it looks like it's for the US only.

fmfool.com (for FM-BC)

and

tvfool.com (for TeleVision)


-Ken

snn47
Posts: 215
Joined: Tue Dec 27, 2016 11:00 pm

Stacked Quad with reflector

Post by snn47 » Sun Feb 12, 2017 9:28 pm

My best advice for your tests is to use a stacked Quad with reflectors. I remember a very light foldable 4 stacked Quad design used for portable HAM Radio contests, for which everything had to by as light as possible, since you had to carry all. I looked through my old UKW-Berichte/VHF-Communication, but could not find it on short notice.

The idea was to have a stacked Quad to improve gain, and reduce the weigth by having a stable horizontal top support and have the rest hanging below as ligthweight as possible, using only thin wires and tubes. The only time I found it on the web is here http://mw0idx.co.uk/quadruplequad.html. I can't recall having read that the antenna was meassured by anyone for performance, but it is simple and lightweight in design, and worked for the HAM that used it at the BBT contest.
Image
The difference in this antenna was that he used it 90° rotated, which is why he neded the thick tubing for support, while when you have it hanging verticallay you need only a fiber glass fishing road or 2 by 4 for support.
I never used it as transmit antennae, but since it is wideband as UHF TV reception. The stacked Quad is directional, works also without the reflecting stacked Quad array, or if you just use 2 instead of 4 elements. Since it was UHF I could use welding rods with a solid cooper coating, all elements soldered together into a self-supporting design.

Instead of coax I used 240 Ohm symetrical cable to feed the driven element for weight restriction. When needed I used a standard TV 240 to 60 Ohm Balaun at the the end of the 240 Ohm symetrical cable before the receiver input.

This antenna is easier to built than solid Quad antennas using tubing. I think this is the easyest way to get started.

Kenn
Posts: 51
Joined: Fri Apr 08, 2016 8:43 pm

Re: Simple dipole for FM-BC reception. To balun or not to ba

Post by Kenn » Mon Feb 13, 2017 10:47 am

That quad looks very intriguing, I've bookmarked for the next antenna project. Thanks for the tip!

-Kenn

DB Gain
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Joined: Tue Dec 20, 2016 8:20 pm

Re: Simple dipole for FM-BC reception. To balun or not to ba

Post by DB Gain » Wed Mar 01, 2017 5:30 pm

If using twin lead as the lead in to the receiver, no balun needed. If using coax I'd use a balun as they should make the transition from balanced (the dipole antenna legs) to unbalanced (the coaxial cable) optimal. A balun should also help reject noise coming off the coax from inside the house from getting to the antenna, but ferrite beads on the coax may do better in that respect. A lot of people run a dipole with coax feed with no balun and say their system is balanced, it's not.

snn47
Posts: 215
Joined: Tue Dec 27, 2016 11:00 pm

Important info Symetrical Twin-Wire-Feeder or Twin-Lead

Post by snn47 » Sat Mar 11, 2017 6:44 am

A few more important information for all not familiar with twin lead cable, which is not necessarily complete:

Symetrical Twin-Wire-Feeder or Twin-Lead was the only (affordable) means to connect an antenna to BC and TV receiver. Since there is not to much that you can find in the internet today her a short Twin-Wire-Feeder or Twin-Lead:

Europe decided on 240 Ohm impedance to match a folded fullwave dipole for twin lead
US decided on 300 Ohm which is decribed in english Wikipedia https://en.wikipedia.org/wiki/Twin-lead

If you use Symetrical Twin-Wire-Feeder or Twin-Lead for building antennaes, like the J-antenna you may have to adjust the dimensions to adjust for 240 Ohm/300 Ohm Symetrical Twin-Wire-Feeder or Twin-Lead cable

- twin lead is unlike coax cable not shielded and therefore not immune
-- to beeing in close proximity to objectes, especially conducting ones. This is why twin lead was mounted on spacers.
-- rain snow or ice.
There were a few twin lead variants produced to improve matters, but I am not aware that you can still buy them today (see https://books.google.co.th/books?id=U9a ... ad&f=false )

- not all Twin Lead are UV light resistant and age with time

- not all BalUn's for BC/TV are broadband, but may contain high pass/low pass filter inside, since older TV in Europe used to have a VHF and a UHF symetrical input (e.g. https://www.amazon.de/Antennenweiche-IE ... ennenkabel ).

- Unless your receiver has a 240/300 Ohm symetrical input to which you can connect 240/300 Ohm symetrical cable directly, you need an BalUn.

- BC/TV receiver mostly had internally a BalUn (Balanced to Unbalanced) that provided a 1 to 4 impedance matching, which was a good place to find them.

You will need a BalUn with 1 to 4 Impedance matching for RTL-SDR Sticks unbalanced/asymetrical coax input whether it's SMA, F or some other type of connector.

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