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My homemade coax dipole antenna


resonate

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Hi all!

I got myself a micron sda 214-1 splitter/amplifier and wanted an external antennae to get rid of the RF noise I was getting from my bag (mostly tascam dr-680's fault) I also use a Sennheiser G3 transmitter as a mono hop/scratch/ifb. Which is surprisingly strong at that task :) Previously i had to hang the receivers on my harness but as i recently bought more channels of wireless, this task was too cumbersome to continue - a lot of cables and mess on the harness. So i thought about lifting only the antennas up.

 

I bought some RG58 cable, and (after a lot of reading) tried to make a simple coax dipole - with stripped cable, but it didn't seem to work. My kit works at the old CH69 (841-865 Mhz) frequencies, we still can use them in Poland. Probably i messed something up with crimping the connectors, as i was trying to have everything nice and tidy and wanted an sma-> BNC extension cable with a socket on the bag that i can hook antennae to.  

 

Then, i found this calculator, and tried the data it gave me : http://www.nsw.wicen.org.au/technical/projects/coaxial-folded-dipole-antenna

 

I used rg58 coax (the calculator calculates the cable loss for you), some pvc pipe and plastic 'thingy' that is used to connect furniture i guess. on the end of the coax, according to the design,  there should be a feed line cable (300 ohm). I didn't have that cable so i just used pieces of steel metal coat hanger and soldered them on the end of each coax arm (the lenght of the 'feed line' is shown inside the calculator).

 

It works! the range got very stable, on the last night testing run i got a healthy 100+ meters from the dipoles connected to micron SDA, antennae mounted on my harness. 

 

Here are the pictures:

 

 

Hope somebody find it useful. I wonder how much better the SNA 600 design is? This was dirt cheap for me, and anyways i can't use SNA600 as not designed to reach CH69 frequencies.

 

 

Bartosz

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[snip]

 

 

 

Hope somebody find it useful. I wonder how much better the SNA 600 design is? This was dirt cheap for me, and anyways i can't use SNA600 as not designed to reach CH69 frequencies.

 

 

Bartosz

Hi Bartosz,

The last setting on the SNA600a is 750-850 MHz which includes ch69 (US). Still, the price on your rig is right.

Best,

Larry F

Lectro

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Thanks Larry, i didn't know that there is an sna600a version available. All i ever see on ebay is sna600 up to 800mhz! 

 

Jon, actually Audio LTD was a source of inspiration, they sold similiar design with RK4 rack. i use big metal paperclips to attach the cable (rg58 is pretty sturdy) to the harness. 

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Thanks Larry, i didn't know that there is an sna600a version available. All i ever see on ebay is sna600 up to 800mhz! 

[snip]

 

 

resonate: " All i ever see on ebay is sna600 up to 800mhz! "

I suspect it is difficult to notice any difference in performance...

I routinely use these antennae (sic) successfully across multiple blocks

Both are correct. The SNA600a added the DC blocking capacitor but the fundamental antenna stayed the same. Now it is marked as going to 850 MHz, which it always did. As Mike says, they are very broadband and will cover 3 blocks with a little loss at the extremes.

Best,

Larry F

Lectro

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Well, that's good to know! maybe i will hunt for some sna600 on ebay then.

 

Both are correct. The SNA600a added the DC blocking capacitor but the fundamental antenna stayed the same. Now it is marked as going to 850 MHz, which it always did. As Mike says, they are very broadband and will cover 3 blocks with a little loss at the extremes.

Best,
Larry F
Lectro

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Larry,

the one thing that puzzles me though, is different diversity switching on the the same model of receivers, tuned to the same frequency, fed by one transmitter. half of the receivers switch between one antenna or the other, and the other half is pretty much sold on the second antenna input, sometimes switching to the first. is there any explanation for this? are there too miniscule differences between antenna 1&2 performance? The receivers&Tx are Audio DX 2020.

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Both are correct. The SNA600a added the DC blocking capacitor but the fundamental antenna stayed the same. Now it is marked as going to 850 MHz, which it always did. As Mike says, they are very broadband and will cover 3 blocks with a little loss at the extremes.

Best,

Larry F

Lectro

Larry.

Is DC blocking capacitor the reason why I can not get rid of the circut board on the SNA600a?

I don't know if you are familiar with my conversations with Gordon about getting rid of most of the circuit board and attaching the BNC  next to the actuall antenna.

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Hi Rado,
The SNA600 is more sophisticated than it appears at first glance. The problems with just hooking a BNC to a pair of dipole ears are several. First, a dipole in free space is  62 Ohms resistive at 0.46 wavelengths or 73 Ohms resistive plus 45 Ohms reactive at exactly 0.5 wavelengths (half wave dipole). The second bigger problem is that a dipole is a balanced radiator, i.e., both arms radiate but out of phase. The coaxial cable feeding the dipole is not a balanced system.
 
If you look at the SNA600a under the coating you can see transmission lines embodied in the copper. Note the width varies. This microstrip line transformer is a quarter wavelength long and transforms the higher impedance of the dipole to 50 Ohms. In addition, note the thick copper sections that are grounded at one end. This forms a 1/4 wave choke balun (BAL-anced to UN-balanced = BALUN) that matches the balanced dipole to coax.
 
Here's a Wikipedia article:
 
Ignore the math, look at the graphs and the drawing of the "Dipole using a sleeve balun." The SNA600 copper choke balun is similar.
 
The fact that the SNA600 (Swiss Navy Antenna) folds up like a pocket knife is just gravy.
Best Regards,
Larry Fisher
Lectrosonics
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Larry,

the one thing that puzzles me though, is different diversity switching on the the same model of receivers, tuned to the same frequency, fed by one transmitter. half of the receivers switch between one antenna or the other, and the other half is pretty much sold on the second antenna input, sometimes switching to the first. is there any explanation for this? are there too miniscule differences between antenna 1&2 performance? The receivers&Tx are Audio DX 2020.

My guess, if the antennas are equal, is that half the receivers have the switching thresholds set to be equal between the internal receivers so they are seeing about the same levels and switching as the antenna levels do their usual dance. The ones that don't switch as often, are adjusted with one receiver's threshold substantially lower so it tends to stay on all the time. As Mike said, I'm not the expert on these ALTD units. The units that are switching fairly often are probably the ones that are set right. If you are not having any dropout problems, I wouldn't worry about it. You said the "lazy" units are switching sometimes so they are working.

 

If you want to test that they are ok, get your transmitters hundreds of yards away (or disconnect the transmitter antennas at 20 feet away) and see if the two types of units are performing roughly the same as far as dropout protection or noise ups with weak signals.

Best Regards,

Larry Fisher

Lectrosonics

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  • 1 year later...

Curious about the balun built into the SNA600a... Is there a practical way to build a balun into a home-made coaxial dipole made from a piece of RG58 or 174? Is a balun more important when using the antenna as a transmitting antenna or does it not matter?

I'm wondering if there is a practical way to make an electrical equivalent to the SNA600a but in a more harness-strap friendly design.

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Curious about the balun built into the SNA600a... Is there a practical way to build a balun into a home-made coaxial dipole made from a piece of RG58 or 174? Is a balun more important when using the antenna as a transmitting antenna or does it not matter?

I'm wondering if there is a practical way to make an electrical equivalent to the SNA600a but in a more harness-strap friendly design.

I tried by modifying a few lectro dipoles but could never get the same performance.

Even just making the antenna straight gave me worse effects. "check the picture on the bottom".

In my opinion if somebody can make a harness friendly dipole it is lector.

Meanwhile a coax antenna in the harness is easy but still not as good as dipole. Don't know if the bigger size or the impedance matching electronics in the lectro dipoles but no antenna comes even close. except a sharkfin.

 

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Edited by RadoStefanov
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