I built a Dipole Antenna, but it's not working.

[DavidLarson52]

I'm brand new to the hobby, and got myself a Uniden SDS200. Tried to make my own Dipole Antenna, but when I put the alligator clip on the radios antenna, there is zero increase in reception across the entire frequency range of the radio. I know I am doing something totally stupid here, so I would really appreciate any help you may provide. Here are a few pictures...


This is 100 feet of generic speaker wire, strung between two big trees. From there the two wires go to an outside deck on my house.

Closeup of how the two wire ends are attached to the trees, using zip ties...



The intersection of the two wires. Just some black electrical tape...



At my deck I've got a lot of extra wire that I simply winded up into a coil...



Here is the end that connects to the radios antenna...



Thanks again for any help you can provide.

 

[John_S]

Way too long for scanner frequencies...maybe ok for shortwave listening. Actual dipole length would only be anywhere from a few inches to a couple feet.

 

[Enforcer52]

Try building one of these if you want a homemade antenna.

Homebrewed Off-Center Fed Dipole - The RadioReference Wiki

wiki.radioreference.com

 

[mmckenna]

You don't have a dipole if you have both conductors connected together at the alligator clip. You've just got a very long piece of wire that's no where near resonate at the frequencies your scanner covers.

It's also horizontally polarized, which is the opposite of what most of the stuff you probably want to listen to is using. Vertically polarized is what you want. That's reducing a lot of the signal right there.

Look up plans on how to build the off center fed dipole (mentioned above) or a 1/4 wave ground plane. You'll want to connect those with actual coaxial cable to the antenna jack, no clipped on to the existing antenna.

 

[KD9KSO]

Don't trash it. Buy a SW receiver.

 

[Ubbe]

Thanks again for any help you can provide.

A dipole have two elements, the ones you have connected between the trees, and will receive a signal but from one element it will be in the opposite phase. If you use speaker wire to transfer the two signals in parallel to each other, the two signals in opposite phases will cancel each other out and you receive nothing. You will need to use a coaxial cable from the point where the two elements meet in the middle.

But the whole antenna are designed for shortwave frequencies and horisontal polarized. But it will probably receive better than any indoor antenna, if you invest in some CATV 75 ohm coax to be used as the downlead to the scanner.

/Ubbe

 

[rivardj]

A dipole have two elements, the ones you have connected between the trees, and will receive a signal but from one element it will be in the opposite phase. If you use speaker wire to transfer the two signals in parallel to each other, the two signals in opposite phases will cancel each other out and you receive nothing. You will need to use a coaxial cable from the point where the two elements meet in the middle.

But the whole antenna are designed for shortwave frequencies and horisontal polarized. But it will probably receive better than any indoor antenna, if you invest in some CATV 75 ohm coax to be used as the downlead to the scanner.

/Ubbe

Not true. The speaker wire from the feed point to the alligator clip will act as a feed line just like open-wire feedlines, however, it will be slightly more lossy than proper twin lead or open-wire feedline. The problems with his setup are that the wires of the feedline are shorted together at the alligator clip, wrong polarization and the dipole is not the correct size for any frequency his scanner will receive as stated by you and others.

Zip Cord Transmission Lines and Baluns

 

[dkcorlfla]

What are you interested in scanning? The higher UHF frequencies need a much shorter antenna.

Check out the ultra simple, cheap and easy to make 1/4 ground plane I just made for a Meshtastic Lora board I playing around with.

I built it for 913Mhz, the NANO was calibrated for 900 >950Mhz. Looks like I missed the mark by just a little. Needed to be a tick longer then 3.1 inches.

Link to dipole calculator:

Antenna Calculator - West Mountain Radio

Also be aware that as you go above low VHF the loss in the coax feed cable starts to be large.

If UHF keep the feed coax as sort as possible.

 

[DavidLarson52]

Wow! Many thanks for ALL of your useful advice, and also for pointing out my mistakes.

I have decided to not build my own antenna, for now. Need to spend a lot of time getting to learn a lot more about scanners, GMRS, and Amateur Ham Radio. Decided to buy myself a Discone antenna for my scanner. Please advise me with your opinion of this setup...



Tram 1410 VHF/UHF Super Discone Antenna:




Coax Cable 20 feet RG8x:



DHT Electronics RF Coax Adapter:

 

[dkcorlfla]

That will be an excellent choice, I have the same discone up outside at about 20 feet to the top. Works great on VHF and ok on UHF.

The 20 feet of RG8X will work ok as long as you don't need to go longer there will be loss in the coax at UHF but it will work good enough to get you started.

Then after you get to scan and find things of interest you could make a antenna for the target frequencies. 1/4 wave ground-plane verticals are easy to make. A J pole can offer even more gain but are harder to make.

Also note that if you ever get interested in ham radio the discone above will work well on VHF 2 meter ham. UHF ham or GMRS not so much.

hope this helps, Dale

 

[KD9KSO]

I used to have one for general coverage when I lived in a different RF intense location. Now I live out in the sticks and I opted for ground planes instead.

 

[Ubbe]

If I look at the attenuation at 900MHz for RG-8X it seems to be 12dB for 100ft compared to RG6, standard sat dish cable, that have 6dB and 1/4 of the cost and easier to route.

/Ubbe

 

[dkcorlfla]

If I look at the attenuation at 900MHz for RG-8X it seems to be 12dB for 100ft compared to RG6, standard sat dish cable, that have 6dB and 1/4 of the cost and easier to route.

/Ubbe

Clipped from Wiki "RG-6/U is a common type of coaxial cable used in a wide variety of residential and commercial applications. An RG-6/U coaxial cable has a characteristic impedance of 75 ohms."

There will be more loss because of the impedance mismatch. To the best of my knowledge the scanner needs 50 ohms.

Also, easier to route the RG-8X? If you buy the good stuff with a stranded center I think one could tie their shoelaces with it.

 

[mmckenna]

Clipped from Wiki "RG-6/U is a common type of coaxial cable used in a wide variety of residential and commercial applications. An RG-6/U coaxial cable has a characteristic impedance of 75 ohms."

There will be more loss because of the impedance mismatch. To the best of my knowledge the scanner needs 50 ohms.

Also, easier to route the RG-8X? If you buy the good stuff with a stranded center I think one could tie their shoelaces with it.

The antenna input on the scanner isn't going to be a perfect 50Ω across the full range of coverage.
The discone may not be a perfect 50Ω design, either.

Also, remember that UHF connectors are not a perfect 50Ω.

Adapters add additional loss, too.

Using good RG-6 will have less loss, and being able to install the correct BNC adapter will reduce connector loss.

Anyway, it's a hobby, and discone antennas have zero gain, so probably not the place to get hung up on a slight loss due to a small impedance mismatch.

 

[DavidLarson52]

Quick question regarding the RG8x coax cable I’ll be using. I know there is signal loss the longer the cable is, but at what length does the signal loss seriously affect the quality of the radio signals received? Thanks!

 

[mmckenna]

Quick question regarding the RG8x coax cable I’ll be using. I know there is signal loss the longer the cable is, but at what length does the signal loss seriously affect the quality of the radio signals received? Thanks!

Coaxial loss is not only impacted by length, but by frequency. So length is only part of the equation, you'd need to know what frequency to properly figure out the amount of loss.

As for how much loss with affect quality, that entirely depends on how strong the signal is at the antenna. If it's really strong, like you are near the transmitter, the coax length won't be as critical as if you were on the very fringes of the radio system coverage.

Unless you are far from the transmitter, 20 feet of RG-8X likely won't be a big deal. Very common to have 10-15 feet of RG-58 (more loss that RG-8x) in a mobile installation.

 

[John_S]

Clipped from Wiki "RG-6/U is a common type of coaxial cable used in a wide variety of residential and commercial applications. An RG-6/U coaxial cable has a characteristic impedance of 75 ohms."

There will be more loss because of the impedance mismatch. To the best of my knowledge the scanner needs 50 ohms.

Also, easier to route the RG-8X? If you buy the good stuff with a stranded center I think one could tie their shoelaces with it.

The SWR mismatch is minimal for receiving purposes...really not relevant till it gets much higher. What's more important is the signal loss in 8X compared to RG-6. 8X is only useful on HF frequencies unless the runs are less than 25' or so. RG-6 will always be better for scanner usage. You can get terminated lengths of quad shield RG-6 right on Amazon.

 

[dkcorlfla]

Looks like I missed the mark on this one. Can you tell I come from an armature radio background? Hams spend too much time fussing over SWR ;-)

I did find a easy to use online calculator and checked RG8X vs RG6A (did not see RG6 and do not know what diff the A makes)
At 800Mhz the RG6A did show less loss at 50 foot run. Looks like either cable would work fine with the edge going to the RG6A.

Coax Calculator

 

[John_S]

Looks like I missed the mark on this one. Can you tell I come from an armature radio background? Hams spend too much time fussing over SWR ;-)

I did find a easy to use online calculator and checked RG8X vs RG6A (did not see RG6 and do not know what diff the A makes)
At 800Mhz the RG6A did show less loss at 50 foot run. Looks like either cable would work fine with the edge going to the RG6A.

Coax Calculator

It's natural to pay extra attention to SWR when you need to transmit and receive on an antenna...especially with coax fed antennas.