Is this a balun?

[Nigel Goodwin]

The Amazon wire antenna is a simple "rabbit ears" dipole with no gain. It works fine with a high quality home stereo receiver but not with a cheap FM radio.

It's simply what I described above - a crude dipole - but without a balun (which would improve it somewhat). In fact, it's pretty well identical to what we used to make when setting things up for customers, a bit of speaker wire, pull it apart to the length of your arms, fit a coax plug on the end - and they want $8.99 + expensive postage?

However, they work just as well on a cheap stereo as an expensive one - if the stereo is crap, then it's not the aerials fault - in fact many stereos come with that same basic wire aerial, and often a (much more useless) AM loop aerial.

 

[audioguru]

He says his FM reception is weak on most channels. Maybe the radio circuit is cheap or maybe he lives in a cave or far from a city.

 

[Nigel Goodwin]

He says his FM reception is weak on most channels. Maybe the radio circuit is cheap or maybe he lives in a cave or far from a city.

As he's not using a suitable aerial, you would expect it to be weak!.

 

[fixit7]

My radio is a Denon. Made in Germany. Not a cheap radio. Some of you crack me up. Next someone will think i live in a Faraday cage.

 

[Nigel Goodwin]

My radio is a Denon. Made in Germany. Not a cheap radio. Some of you crack me up. Next someone will think i live in a Faraday cage.

You pretty well do if you're trying to feed it from a UHF aerial

I take it you've not tried my simple speaker wire aerial?.

 

[audioguru]

I was given an "FM radio" from The Dollar Store that uses a Chinese copy of a Philips scanning FM radio IC. It has a little whip antenna that is too short for good FM reception and the radio's speaker is so small that it sounds horrible. It picks up most of the many FM stations in my city and the clock radio that I bought for $1.75 in a surplus store also picks up most stations with its antenna built into its AC cord.

Denon (Nippon DENki ONkyo) is a Japanese company and has made high quality audio and video products there for many years. Their radio should pickup many FM stations with just a piece of wire for an antenna. Maybe it is defective.

 

[Nigel Goodwin]

Denon (Nippon DENki ONkyo) is a Japanese company and has made high quality audio and video products there for many years. Their radio should pickup many FM stations with just a piece of wire for an antenna. Maybe it is defective.

He's not had an aerial connected to it, not as good as a piece of wire anyway - which probably explains it all.

 

[unclejed613]

Could you give more details on how to scale it 3:1?

i was talking about the antenna dimensions, not the balun....
you don't need the whole phased stack. the phased stack is useful for UHF because above 300Mhz radio waves don't diffract over the horizon as well as VHF. anything above about 700Mhz is going to be completely line of sight, from 300Mhz to 700Mhz there is some diffraction over the horizon as well as diffraction around small obstacles. at 100Mhz where FM stations live, there's more diffraction around the curvature of the earth, as well as diffraction around larger obstacles.

a regular dipole is two wires fed at the center, with each wire 1/4 wave long. the impedance of this dipole is usually between 30 and 90 ohms, although people who design antennas try to get as close to the industry standard of 50 ohms as possible. with TV antennas, the industry liked 75 ohms for some reason (not just for RF, but for baseband video signals as well). a 75 ohm cable and 75 ohm antenna will give a 1.25:1 SWR match to a 50 ohm radio, so it's no big deal to mix 50 and 75 ohm stuff. an older industry standard for the antenna and tuner of TV sets was 300 ohms. a folded dipole (take the original dipole, and run another piece of wire across it connecting the outside ends together) presents a high impedance at the feedpoint, 300 ohms for TV antennas. the reason this was used in the past was 300 ohm cable was easy to make, and used less copper per foot than coax cable. the TV tuners having a 300 ohm input impedance were more tolerant of impedance mismatches caused by the folded dipole receiving signals that were not at the center frequency of the antenna. if you use a single "bowtie" antenna with each element 75cm from the center where the elements are bent in half (so each element uses 150cm of wire) to the ends of the elements, the feedpoint impedance will be between 40 and 80 ohms. the "bowtie" shape widens the bandwidth of the antenna because it acts like a wire with a larger diameter. in essence it's a biconical antenna.

 

[Ylli]

I've flipped about 25 receivers since Christmas, and it seems to me the mid range units have the best tuners. Got a 1988 Kenwood on the bench currently, and it has a very hot tuner, both AM and FM. It gives me full quieting with one of the weakest station I test with using a 12" piece of wire. Conversely, the couple of H/K's I've worked on have middle of the road tuners.

 

[Ylli]

a regular dipole is two wires fed at the center, with each wire 1/4 wave long. the impedance of this dipole is usually between 30 and 90 ohms, although people who design antennas try to get as close to the industry standard of 50 ohms as possible. with TV antennas, the industry liked 75 ohms for some reason

My understanding is that 75 ohm is lowest loss, but 50 ohm has best power handling capability. So if you are receiving only, 75 ohms is the way to go. If you are transmitting and need the feedline to handle significant power, then 50 ohms is a better choice.

 

[unclejed613]

He's not had an aerial connected to it, not as good as a piece of wire anyway - which probably explains it all.

when i worked for a service center, i got a lot of receivers coming in where the complaint was "audio cuts out on FM". the cause 99 times out of 100 was there was no antenna connected. i only replaced one Denon tuner in 9 years at the service center, and that was because the F connector was broken off.

 

[audioguru]

First he used the balun as an FM antenna.
Then he connected a UHF TV antenna (maybe to the balun that hopefully is connected to the radio?).
I wonder if the radio has unbalanced 300 ohms antenna screws or if it has a 75 ohms coax connector?