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Mar 13, 2015
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Hi! i would really appreciate a little help on the topic. i found myself baffled about a few things concerting my antenna. so i ordered an FM transmitter (50mW-500mW). its output impedance is 50 ohm. i have an old antenna just like this http://2.bp.blogspot.com/_Hi6MH5nm2jc/S9wsX-4IycI/AAAAAAAAAGA/lU_IfKBC5Ug/s1600/image006.png and it has an in-built balun of 50ohm-300ohm. (1:4). now from what ive read a dipole antenna should have an impedance of around 50ohm. why is the balun rated at 1:4 with an output of 300ohm. i also read that the balun ill need should be 1:1 current balun. can i get some quidance on how to build a 1:1 balun for 87Mhz-108Mhz.

#### Y2KEDDIE

Sep 23, 2012
259
The indoor, set top, antennae you have is a dipole type, with a transformer or resistor network to match a nominal 75 Ohm to 300 Ohm. The cable coming from the housing appears to be 300 Ohm balanced line.
*The use of balanced (symmetrical feed) or unbalanced (unsymmetrical feed) creates different characteristics
I say 75 ohm nominal, because when the two rods are extended parallel with the earth under ideal conditions , extended to the correct length (resonant frequency desired, the impedance is 75 ohms at the center connection. If you slope the rods as in an inverted V configuration the impedance decreases. If the v forms a 90 degree angle (sloping downward), the impedance is closer to 50 ohms.
A balun was used with older receivers with a folded dipole which has an impedance of 300 ohms , but balanced. A connection with low loss 300 ohm ribbon cable (balanced Line) could be made directly with the advantage of low loss, low cost , and common mode noise rejection.
Newer receivers were designed to have coaxial line (unbalanced) connections for use with Cable TV, etc. In order to get a folded dipole to operate correctly a balanced to unbalanced matching impedance transformer is needed. Hence BAL (anced) to UN balanced; BALUN.
Most likely your transmitter requires a 50 Coax connection (unbalanced). You could connect a 300 to 75 ohm BALUN backwards to the 300 ohm cable of your antenna. Feed the 75 Ohm connector with a 50 ohm cable from your transmitter. There would be some loss because of the transformers but it should work. You could droop the rods to get the inverted Vee configuration and 50 ohm impedance.
This is a theoretical set-up. Optimum results will be from experimenting. There are a lot more factors involved.
Another configuration would be to remove the network/transformer from your antenna and connect a 50 om coax directly to the rods. Center conductor to a vertical positioned rod, and the shield to a horizontal positioned rod (ground plane). The resonant frequency will be determined by the length the rods. Frequency/length is not all that critical as the bandwidth is determined by the diameter of the rods, and at VHF frequencies It’s quite broad.

Mar 13, 2015
5
So if i were to remove the balun altogether and directly plug the 50ohm cable to an L shaped antenna as your last recommendation are there any adverse effects that might occur? The rods are quite thick so if i center them on 70ish cm (the half wave wavelenght of the center frequency i desire) would i get the whole (87Mhz-108Mhz) band? if done so, will 75ohm and 50ohm cable make a difference becauae i have an rg6 which is 75ohm

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#### davenn

Moderator
Sep 5, 2009
14,040
Hi! i would really appreciate a little help on the topic. i found myself baffled about a few things concerting my antenna. so i ordered an FM transmitter (50mW-500mW). its output impedance is 50 ohm. i have an old antenna just like this http://2.bp.blogspot.com/_Hi6MH5nm2jc/S9wsX-4IycI/AAAAAAAAAGA/lU_IfKBC5Ug/s1600/image006.png and it has an in-built balun of 50ohm-300ohm. (1:4). now from what ive read a dipole antenna should have an impedance of around 50ohm. why is the balun rated at 1:4 with an output of 300ohm. i also read that the balun ill need should be 1:1 current balun. can i get some quidance on how to build a 1:1 balun for 87Mhz-108Mhz.

single wire (line) dipoles actually have an impedance of 75 Ω
folded dipoles have a 300 Ω impedance

Yes if it has a BALUN, take it out and connect the coax directly to the antenna terminals

your antenna shown is what is called a Rabbit's Ears" antenna they are made for the TV receive use.
It will work on the FM band. and I will work as a transmit antenna. You are just going to have a mismatch
namely 25 Ω
it will just reflect a little bit of the TX power back down to the transmitter

Dave

Mar 13, 2015
5
okay thanks. really helpful!!! i just have a few morr questions. how are the rods supposed to be oriented to match the 75ohm cable impedance and since my transmitter is rated 50ohm output will there be any problems with the 75ohm cable. this power reflection you are mentioned is it dangerous for the transmitter in this case.

#### davenn

Moderator
Sep 5, 2009
14,040
at the low levels of power you are running, no its not harmful to the transmitter

as far as the antenna rods go, have then as horizontal as possible, then orientate the whole antenna either horizontal or vertical to suit the antenna polarisation of the receiver antenna

Mar 13, 2015
5
thank you . i have one more question a little beside the point. how much range do you think I'll be able to cover with 500mW (in urban terrain)? better yet can you give me a rule or a formula from which to determine the approximate range of a given power level . i think 500mW is not much so oriented myself to ~5W. what might be the range of 5W?

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#### davenn

Moderator
Sep 5, 2009
14,040
500mW could easily get 1 - 2 km in an urban area and since you want to do this on the FM band
you would be operating illegally and would risk the imposed penalties from the radio licencing authority in your country. Im sure you don't want to risk huge fines and or imprisonment ?

Non-licenced transmitting devices are supposed to be low power and short range. If you start interfering with your neighbours FM radio reception, they would have every right to complain to the authorities

Mar 13, 2015
5
did i say urban? excuse my english, sometimes i mix words up.i meant rural. i don't plan on using this in my town since even on 200mW i wouldn't last a week before my version of the FCC sniffs me out. we have watchtowers here and there which will triangulate my location very fast. i plan on using this in my countryside village which is mostly flat fields with the occasional hill here or there. the reason I'm asking is because my village is a mere 20km from the capital city and i wouldn't want to reach it . the terrain i want a good reception is 3-5km. So disregarding the legal limits on my personal responsibility, what output power would you recommend that meets my requirements? once again thanks in advance.
p.s. i watched a youtube video and the guy in the video states that 7W covered him 10km of range which will somewhat fit my requirements.
p.p.s. i don't plan on jamming anyone. we have a few gaps in the FM spectrum that i would happily fit in without anyone paying me any attention.

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#### davenn

Moderator
Sep 5, 2009
14,040
sorry

knowing you are doing things illegally
Im not willing to help you any further, else I could also risk prosecution for the supply of information to commit a crime

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