Balun needed for coax fed dipole on hi-z port?

Useful information regarding antennas for SDR products.
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Balun needed for coax fed dipole on hi-z port?

Postby rack201 » Wed Dec 28, 2016 11:09 pm

Hi - I'm very much a beginner here, and plan to build an inverted-v dipole for use with my SDRplan RSP2.

I understand the hi-Z port has an impedence of 1k Ohms. Does this mean I would not require a balun if using a relatively short (say 10-20 meters?) coax feedline to the dipole?

Alternatively if I were to use balanced 450ohm ladder line it would not be required?
Last edited by rack201 on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Re: Balun needed for coax fed dipole on hi-z port?

Postby g1hbe » Thu Dec 29, 2016 11:45 am

That's about right. On a coaxial feedline which is relatively short, you can take the coax straight into the Hi-Z input. You may get slightly more signal with a 9:1 balun, but if you can see background noise when you plug the aerial in there's not much point in 'more signal' as the noise will come up as well. I use a Wellbrook loop (50 ohm coax fed) with very good results. A 450-ohm line will work well too.
If you are using a coax feeder, try experimenting with which terminals you attach to. There's Ground (you can put the coax braid to this) then you have a choice of P or N for the inner. In theory I think these two should give identical results. Or you could try connecting the braid to N and the inner to P. Then try reversing the connections to see which gives the best signal to noise ratio.

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Re: Balun needed for coax fed dipole on hi-z port?

Postby n1kgy » Sun Jan 29, 2017 2:31 pm

For receive purposes only, 400/450 Ohm transmit rated parallel line is total overkill, and weighs much more than some of the alternatives - There are still supplies of the thinner, and thus lighter, 300-Ohn "TV Twinlead" available, and I highly recommend it for LF~VHF-Lo (50MHz) wire receive antennas.

I actually made a portable/temporary use G5RV using regular old TV Twinlead and some generic 24ga "speaker wire", about a decade ago, and it has worked quite well with my QRP/Portable radio setup. I use "5Lb test" nylon fishing line as the support "rope" and don't even bother to use insulators (the fishing line does more than well enough to insulate the ends of the antenna at QRP transmit levels <=5.0 watts) thus, for RX-only no insulators is definitely no problem if you use fishing line to support the antenna.

Even more interestingly, typical 24ga "speaker wire" with a clear vinyl jacket can be used as a balanced/parallel feedline for LF/MF/HF receiving (and QRP transmitting). Many years ago I read where someone had stated that this was the case, and decided to try it out - I used a roll of Radio Shack 24ga. speaker wire for both the feedline and antenna wire for an HF dipole on 20 meters (just as the article's author had specified) , and it worked very well. at the "center connector" position I just tied a knot in the wire, and separated the two conductors up to that point to make the antenna, and then spooled off another (1/2 wavelength X Velocity Factor which I estimated at 0.80). At that point I cut the wire, wound as many turns as would fit onto an FT-50-43 ferite core to make a balun, and soldered a BNC connector on the end to connect it to my radio (a Norcal Sierra). Subsequent testing with an MFJ-259B antenna analyzer showed the "24ga Radio Shack Speaker Wire" to have a characteristic impedance of 74~81 ohms, and a velocity factor between 0.80 and 0.85 - both the impedance and velocity factor are frequency dependent, but this really only matters for transmitting, where losses are much more noticeable as frequency is increased - for receiving purposes it is perfectly fine for all of MF & HF, and still very serviceable at 50MHz.

Regardless of what the nominal impedance of the feedline is - 80 (speaker wire), 300, 400, 450, or 600 ohm - it's a balanced line and thus you don't need a balun going into the radio's balanced antenna port.

With coax (50 or 75 ohm) however, you probably should use a receive balun - not to increase signal, but to REDUCE NOISE.
Even if you are getting sufficient signal into the antenna port without one, a balun will substantially reduce noise transfer from the outside of the coaxial shield into the antenna port, so you'll have a much lower noise-floor, and thus be able to hear weaker signals.

The difference a good balun makes can be profound - as much as 20~30dB reduction in noise in a typical residential home environment where you have multiple TVs, computers, & etc. all of which are "unintentional radiators" of RF energy into both the air and the wiring of your home. using a balun, and grounding the shield of the coax right at the balun, is necessary. You will get the best results when you have no galvanic path between the coax and the antenna port - so use two totally separate windings on the toroidal core for the balun - the antenna input port should receive 3X or 4X as many turns as the winding which connects to the coax - i.e. if you have 3 turns on the coax side, then the secondary should have either 9 or 12 turns (the transformer ratio is the SQUARE of the turns ratio - so 3 Squared = 9X and 4 Squared = 16X, thus 50 ohms and a 3:1 turns ratio creates a 450-Ohm balanced output, while 50 ohms and a 4:1 turns ratio creates an 800 ohm balanced output from the balun transformer.

DO NOT "GROUND" the COAX SHIELD to the GND Terminal on the green balanced connector of the SDRplay! Ground it to a cold water pipe or electrical outlet ground terminal (to prevent static buildup and reduce the possibility of a nearby lightening discharge frying your SDRplay), but do not connect the coax shield directly to the SDRplay receiver at all.


Last edited by n1kgy on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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