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

Useful information regarding antennas for SDR products.
rack201
Posts: 13
Joined: Mon Dec 26, 2016 12:39 am

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?
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g1hbe
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Location: Cheshire

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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Andy

n1kgy
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Joined: Sun Jan 29, 2017 2:10 am

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.

Cheers

Chuck
n1kgy
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ekendraed
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Joined: Sun Apr 16, 2017 5:52 am

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

Postby ekendraed » Sun May 07, 2017 3:03 pm

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vk7jj
Posts: 36
Joined: Thu Mar 02, 2017 9:56 am

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

Postby vk7jj » Mon Sep 04, 2017 7:13 am

Hi Chuck, I wanted to commend your reply in particular

The difference a good balun makes can be profound - as much as 20~30dB reduction in noise


It sure can and it's a thrill to achieve such an improvement cheaply and easily.

The word "balun" can cause confusion so I thought the simplest thing to do was show-n-tell with a cheap and easy example I use:

Image

• the toroids (Australia source) are cheap and easily obtainable from Jaycar, they have excellent HF performance even below 1MHz, eg.
https://www.jaycar.com.au/l8-25x15x10mm ... 4/p/LO1234

• the diodes (Australia source) eg. 1 amp 1000 volt high speed 1N4007 or similar
https://www.jaycar.com.au/1n4007-1a-100 ... 4/p/ZR1007

• the wire is salvaged from an old CAT5 ethernet tail

The two diodes are across the coax, this clips voltage spikes as a protection for the SPR, I've not noticed any cross-mod effects.
The coax not only connects to the two diodes but to the two wires on the left of the left hand toroid.
The left toroid is a common mode choke, helping get rid of unwanted RF on the coax braid.
The output of that toroid is connected to the two red wires, being the input of the right hand toroid.
The right toroid is a galvanically isolated transformer, upping the impedance to something better than 50 ohm for the HI-Z input.
The two yellow output wires of the right toroid go to the RSP's Hi-Z connector via the black and white wires.

Given many people are using a non-resonant random antenna over a wide range of frequencies with a random length of coax rather than a tuned antenna on a ham band, the actual impedance at the end of the coax could be anything, so it's all non-critical except for the benefit of the galvanic isolation between the two windings.

The really important part for anyone new to radio and wanting to lower their noise floor is to read as much as they can about common mode RF interference and common mode chokes for radio hams.

What fun!

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