Antenna orientation question

[scan_nepal]

Okay I found this dipole TV antenna lying around and hooked up a coaxial cable along with BNC Jack. Reception has improved. What is the best orientation for such antenna? I live about 5 miles from the airport with lots of concrete houses in between. Thanks.

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[zz0468]

Probably vertical, but unless that antenna is cut for the frequency range of interest, it's essentially just a random chunk of metal. Now, random chunks of metal can work just fine as receive antennas, but it probably doesn't matter so much how it's oriented. See what works best, and leave it that way.

 

[scan_nepal]

Probably vertical, but unless that antenna is cut for the frequency range of interest, it's essentially just a random chunk of metal. Now, random chunks of metal can work just fine as receive antennas, but it probably doesn't matter so much how it's oriented. See what works best, and leave it that way.

Yes you are right about that. I tested on various orientation and didn't make much difference. When I had just a rubber ducky, the difference was prominent. I will just leave it the way it is. It's fun listening to complete comm between tower and birds.

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[cmdrwill]

Also your antenna should be away from anything like the patio ceiling. and mounted vertically.
That looks like a VHF folded dipole but may be for 150, the most common.

What does the loop measure end to end?
Most folded dipoles have a impedance matching cable and coax connector 'pigtail'. I do not see that in your photo.

 

[majoco]

Here's mine...can't remember how long each arm is, but I probably cut it for 1/4 wave at 125Mhz or thereabouts. Prcguy put me right on one thing - I had a metal clamp at the centre point (not the cable termination point) but he said that makes two 75ohm elements in parallel. I found a plastic support and then just shorted the two ends together - much more signal - there's a 300 to 75 ohm transformer in the orange box and RG6 quad down to the receiver.

 

[WA8ZTZ]

Most folded dipoles have a impedance matching cable and coax connector 'pigtail'.

this ^^^

 

[WA8ZTZ]

there's a 300 to 75 ohm transformer in the orange box

again ^^^

use of a matching transformer should improve your reception

 

[scan_nepal]

Okay so basically the way I have got the coaxial cable hooked up is not optimal. Dipole ends were hooked with each wires from coaxial. So if I understand correctly, the main issue is the impedance off of BNC jack is not the same one as what the scanner accepts!

So I need to make sure the impedance is changed by using balun. My question is how do I measure the value? And next question is where does it get installed (near the antenna or near the scanner)?

Sorry this maybe a basic question but I am a complete noob in this antenna field. I want to learn more since even the best radio is worthless without a decent antenna! Thanks again all.

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[jwt873]

The folded dipole TV antenna should have an impedance of 300 ohms. Your scanner should have an input impedance of 50 ohms. So, you'd need a 6 to 1 balun.

Most TV baluns transform from 300 ohms to 75 ohms.. But 75 ohms is close enough to 50 ohms so you don't have to worry about it.

The balun should go right at the antenna feedpoint (most are designed to go there). Look at majorco's post above. Note the orange block he refers to.

They come in different shapes and sizes. You can purchase them from eBay 2x Outdoor Antenna 300 To 75 Ohm Coaxial Cable Matching Transformer UHF/VHF/FM | eBay

As zz0468 points out, any chunk of metal will work, but note, if you want the antenna to be optimum, it has to be resonant at the frequency you're receiving. The aluminum loop should be 1.13 meters from tip to tip. (This is the resonant frequency for a dipole at 125 Mhz).

 

[scan_nepal]

again ^^^

use of a matching transformer should improve your reception

Would you mind posting a close up picture of the balun connected to the dipole if possible? I think I can purchase a balun at local TV shop. Thanks bunches!

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[scan_nepal]

The folded dipole TV antenna should have an impedance of 300 ohms. Your scanner should have an input impedance of 50 ohms. So, you'd need a 6 to 1 balun.

Most TV baluns transform from 300 ohms to 75 ohms.. But 75 ohms is close enough to 50 ohms so you don't have to worry about it.

The balun should go right at the antenna feedpoint (most are designed to go there). Look at majorco's post above. Note the orange block he refers to.

They come in different shapes and sizes. You can purchase them from eBay 2x Outdoor Antenna 300 To 75 Ohm Coaxial Cable Matching Transformer UHF/VHF/FM | eBay

As zz0468 points out, any chunk of metal will work, but note, if you want the antenna to be optimum, it has to be resonant at the frequency you're receiving. The aluminum loop should be 1.13 meters from tip to tip. (This is the resonant frequency for a dipole at 125 Mhz).

Started making more sense now! I remember hooking up flat twin lead cable from rabbit ear antenna and connecting balun near TV input back in the day! Always wondered what the heck that was for!

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[majoco]

300:75 ohm TV balun on a binocular core. 300ohm twinlead on the right, "F" coax connector on the left

 

[dsalomon]

Re: coax, if you have a 300 to 75 ohm transformer, then quad shield 75 ohm coax is perfect, but only if you're using for receiving only. A lot of cable companies will give you spare, unused 75 ohm quad shield coax. It's really, really low loss coax, especially with the quad shield. I found an entire spool of it just laying at the side of the road not 100 feet from the Comcast truck. I asked the guy about it and he said to keep it, that they had no need for it. It's fantastic coax for my small home-brew receiving loop.

 

[scan_nepal]

300:75 ohm TV balun on a binocular core. 300ohm twinlead on the right, "F" coax connector on the left

I went to more than few stores to look for the balun. All except for one carried it. This is what it looked like.

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