RadioReference on Facebook   RadioReference on Twitter   RadioReference Blog
 

Go Back   The RadioReference.com Forums > HF / MW / LW Monitoring > Receive Antennas (below 30MHz)


Receive Antennas (below 30MHz) - For all topics related to receive antennas used on HF, MW, LW, etc. For transmit antennas use the Amateur Radio Antennas forum.

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1 (permalink)  
Old 03-09-2017, 9:19 PM
majoco's Avatar
Member
  Shack Photos
Shack photos
 
Join Date: Dec 2008
Location: New Zealand
Posts: 2,419
Default Common mode choke - necessary or not?

I have an OCFD 46feet one side and 23 on the other feeding into a commercial (ex a TTFD) balun of unknown ratio which seems to work quite well. Given that my friend invergordon made a dipole for the 8MHz aero band with excellent results I thought I'd give it a try.

Quarter wave at 8.8MHz is about 28ft - knock off 5% for end effect and you get 26.5 feet a side for the dipole and I made a common mode choke from what I thought was a FT50-43 toroid -11 turns each side. Results - abysmal - even at 2100local which usually has signals booming in - checked with the OCFD and all good.

Daylight and dropped the antenna down and made the choke into a 1:1 balun - still no good. Soldered the dipole ends directly to the coax and whoopee - heaps of signals. Perhaps the toroid is not FT50-43 after all.

Ordered an FT140-43 and some miniature coax to make a guanella choke.

Now this is going to be for receive only, so is a choke beneficial? Does it prevent signals/interference being picked up on the outer of the coax and being transferred to the inner? I feed the coax into an active multicoupler (Reaction Instruments 409-2) which has filter and a 50ohm load right at the input.

Any insights would be appreciated.
__________________
Cheers - Martin ZL2MC - Palmerston North
Reply With Quote
Sponsored links
  #2 (permalink)  
Old 03-09-2017, 10:36 PM
Member
   
Join Date: Jun 2006
Posts: 6,025
Default

If your making a classic OCFD then you should make sure your balun is a 4:1 and a current type is preferred. If your balun is from an old T2FD then it could be a 9:1 or 12:1 depending on mfr.

I don't know what this balun topology is called but from my research this is the very best design in a 4:1 balun for OCFD use: BL-41W-2K-4:1 - MyAntennas.com You can make them yourself but its recommended to have an inductance meter to insure the to halves are wound exactly the same and the cores are matched. I don't have the exact formula for this type balun so my main HF antenna these days for amateur and SWL use is a commercially made "MyAntennas" 133ft long 80m version of the OCFD.

The second best balun type seems to be this arrangement shown in this Elecraft manual and its performance is spectacular with very good common mode rejection and a great match to 200 ohms from about 500KHz to 55MHz. http://www.elecraft.com/manual/E7400...%20Rev%20B.pdf I've made several for portable OCFDs and they work very well. The ferrite core in this case is a BN-43-7051 which will handle up to about 150W continuous and I've also built them with a smaller BN-43-3312 core with very similar results. Either core would be fine for SWL receive purposes.

Getting to your question of common mode chokes, since an OFCD is off center fed, its inherently unbalanced and will force some currents onto the feedline. The poor souls who use a voltage balun like the Unadilla W2DU balun with no common mode choke on the feedline will enjoy lots of hot RF in the shack when used on an OFCD for transmitting. Not to mention the possibility of RFI from things in the shack traveling up the coax to be picked up by the antenna.

Now here is my noise story. My previous HF antenna was a ZS6BKW which is a cousin of the G5RV. It needs a 1:1 balun at the junction of the coax/balanced line and I had a 1:1 choke balun but of questionable quality. Not knowing I had an RFI problem and knowing what most HF bands look like on my radio's spectral display, I replaced the questionable choke balun with a very high end one with huge resistive choking loss all across the HF band. Its basically this model but I hand made one to the closest specs I could determine using two different types of ferrite and two large 2.4" cores per side: CMC-230-5K - MyAntennas.com

After swapping out my existing 1:1 choke with the big elaborate one I immediately noticed an overall several dB reduction in my noise floor on my spectral display with many birdies gone and others significantly reduced. I never knew I had this RFI problem and I had been running a 1:1 choke balun, this reduction in noise was from simply upgrading my existing choke balun to a much more effective one.

It turns out my radios sit next to a computer and monitor with the feedline running across them and in parallel with various router cables, USB cables, switching wall wart supplies, etc. Some noise from all these components were being induced onto my coax shield and traveled up to the antenna where it was radiated into the antenna and back down to my radio. I have since put very effective 1:1 choke baluns at both ends of every HF feedline I have to insure no noise is picked up and transferred to my antennas via the coax shield.

So to answer part of your question, yes, an effective choke can be very beneficial in a receive only system and it can prevent signals traveling on the outside of your coax from getting into your antenna.

You can make a fairly effective choke balun for the HF bands with really good resistive choking in the roughly 4 to 15MHz range by wrapping 12 turns of RG-58 size coax around an FT-240-43 core. Or you can get serious and wrap 10 to 12 turns around a pair of FT-240-31 cores, then 10 to 11 turns around a pair of FT-240-52 cores close together on the same feedline.

Each set of cores will favor different parts of the band and the resistive choking impedance will go way up where they overlap. If you use RG-142 Teflon coax instead of RG-58, these dual core chokes will easily handle 5kW of power on transmit if you are so inclined.

Here is a good chart to help select the core type and amount of turns to achieve the highest resistive choking impedance for specific frequency ranges: Common-mode chokes
prcguy

Last edited by prcguy; 03-09-2017 at 10:44 PM..
Reply With Quote
  #3 (permalink)  
Old 03-10-2017, 10:19 AM
Member
   
Join Date: Jun 2006
Posts: 6,025
Default

I just noticed your choke balun problems were with a conventional 1/2 wave dipole and not the OCFD, so disregard the first half of my last post.

If you were simply winding coax through a ferrite toroid then it should not have made that much effect on your reception. Either the core material is appropriate to provide resistive impedance across the frequency range you need, or its just some more coax in line. It should not have had a drastic reduction in your reception if its the wrong mix.

Unless maybe you were transitioning to wire around the core or something other than simple wraps of coax....
prcguy
Reply With Quote
  #4 (permalink)  
Old 03-10-2017, 9:56 PM
majoco's Avatar
Member
  Shack Photos
Shack photos
 
Join Date: Dec 2008
Location: New Zealand
Posts: 2,419
Default

Thanks for all that info. Next time I take the OCFD balun down I'll try to work out the ratios. Previously I made a 9:1 trifilar transformer on that same FT43(?) core and although It seemed alright, when the T2FD balun became available and fitted, I was pleasantly surprised at the increased activity. My reasoning behind thinking it was a 9:1 is that the load resistor on the T2FD was 450ohms.

Anyway, the questions about the choke were based on the apparant lack of performance with the FT43 core - I wound 12 turns bifilar of enameled wire quite widely spaced around the core as per most of the info that I found and was surprised about the lack of signals when compared with the OCFD. The FT140-43 core was reasonably cheap, the coax was the expensive part but it can all go in the junk box if it doesn't work.

At the moment both antenna have almost the same sensitivity give or take a few dBm. I have a tame signal on 13.560Mhz that I use for a standard - it seems to be quite constant. I don't know where is comes from, that frequency is in the ISM band.

The Elecraft choke is interesting - those ferrite blocks are available from Mini-kits in Australia quite cheaply and ex-stock.

I'm hoping to get a reduction in noise pickup - most of mine seems to come from a TV somewhere local - comes on regular as clockwork at three pm when the kids come home from school and goes off around 2230. Interestingly I did get a wavering whistle on a local broadcast band AM station, seems to be on a slightly different frequency but within the passband whenever it came on. Conferred with the nieghtbours and they got it too. Started sleuthing with a portable but the loudest signal was from the phone distribution box down the street so that was a dead end for me. Called the Telecom people and they were happy to investigate as it interfered with broadcast radio. They tracked it down to house about 1/2 mile away with a big stereo that had a cracked ferrite core in a flourescent display driver and was oscillating quite happily unknown to the owners as they didn't listen to that radio station! The Telecom guys even took the stereo away and repaired it - how's that for service!
__________________
Cheers - Martin ZL2MC - Palmerston North

Last edited by majoco; 03-10-2017 at 10:08 PM..
Reply With Quote
  #5 (permalink)  
Old 03-10-2017, 10:31 PM
Member
   
Join Date: Jun 2006
Posts: 6,025
Default

So your choke that was not performing was simply wound with 12 turns of a parallel wire transmission line? That should not have caused a problem unless the core was some super high permeability one intended for switching power supply or AC mains frequencies. In that case maybe the core had a bad interaction with the bifilar windings? I know if you use coax for the windings there will be no adverse effects from using the wrong ferrite mix, you will just not get the choke balun performance you were hoping for.

I used to wind 1:1 choke baluns with parallel wire transmission lines made of 14ga enameled wire and the only concerns there are keeping the wires of the transmission line very close together to keep the impedance low but leave some space between the windings. If you used enamaled wire in the 16 to 14ga range the two side by side wires are a bit higher than 50 ohms and if you leave some space between them you can get into the 200 ohm range before you know it.

That would generally not cause a problem at 8MHz but at 30MHz where the length of wire needed to make 12 turns is now a measurable fraction of a wavelength, the 1:1 balun will now start to be an impedance problem in a 50 ohm coax system. Its also a good idea to wrap the core or feed the transmission line wires through some Teflon tubing to give them some isolation from the core for HV arcing purposes.

The last and very last 1:1 balun I made with bifilar windings was on a T200 powdered iron core that's been part of a balun kit sold for the last 50yrs. I didn't wrap the core and I used it between the coax and ladder line junction of a G5RV. I lit it up one day with 1500w and the wires arced to the core, blew a chunk out and started a fire in the plastic box it was housed in. Since then I wind 1:1 choke baluns using Teflon insulated RG-142 coax.

The Elecraft model is great for making a 4:1 and on their site you can find instructions for winding it as a 1:1 but I don't know how good it is with respect to resistive choking impedance. There is plenty of technical info on using toroid cores for 1:1 choke baluns, so I will stick with those for for now.

I wish our FCC who is responsible for radio interference would respond to complaints and working on someone's equipment would be completely out of the question. I doubt if the current FCC field engineers could repair anything if their life depended on it.
prcguy

Quote:
Originally Posted by majoco View Post
Thanks for all that info. Next time I take the OCFD balun down I'll try to work out the ratios. Previously I made a 9:1 trifilar transformer on that same FT43(?) core and although It seemed alright, when the T2FD balun became available and fitted, I was pleasantly surprised at the increased activity. My reasoning behind thinking it was a 9:1 is that the load resistor on the T2FD was 450ohms.

Anyway, the questions about the choke were based on the apparant lack of performance with the FT43 core - I wound 12 turns bifilar of enameled wire quite widely spaced around the core as per most of the info that I found and was surprised about the lack of signals when compared with the OCFD. The FT140-43 core was reasonably cheap, the coax was the expensive part but it can all go in the junk box if it doesn't work.

At the moment both antenna have almost the same sensitivity give or take a few dBm. I have a tame signal on 13.560Mhz that I use for a standard - it seems to be quite constant. I don't know where is comes from, that frequency is in the ISM band.

The Elecraft choke is interesting - those ferrite blocks are available from Mini-kits in Australia quite cheaply and ex-stock.

Last edited by prcguy; 03-10-2017 at 10:36 PM..
Reply With Quote
Sponsored links
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -5. The time now is 5:03 AM.


Powered by vBulletin® Version 3.8.2
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
All information here is Copyright 2012 by RadioReference.com LLC and Lindsay C. Blanton III.Ad Management by RedTyger
Copyright 2015 by RadioReference.com LLC Privacy Policy  |  Terms and Conditions