Off Center Fed Dipole Interference issues
G'day all, I'm in need of some assistance.
We have recently constructed a new off centre fed dipole, which is working quite well at the moment (talked to the Ukraine from Sydney Australia last night on 20m).
However we are having a small interference problem and would like some advice.
On 20m everything is fine; I can use my full legal 100w without any issues. However drop down to 40m or 80m and we lose all the signal on our TV for any channel (digital it just dies, analogue it cuts to snow) (FYI digital TV frequencies here are 150Mhz to 250Mhz give or take)
At this stage we believe that it is only our TV that is being affected, but have not been able to confirm this either way.
So yes, any thoughts on what might be happening and how to fix it would be more than appreciated.
Some details that might help:
Running an IC-7000 through an old SWR meter up to the roof into a 4 to 1 voltage balun at the feedpoint of the antenna.
- total length: 122ft
- short side: 41ft
- long side: 81ft
Thanks and Regards
Originally Posted by VK2LBP
Assuming it's not simply a problem of the proximity of the antenna elements to the house, it may be a problem of common-mode current on the feedline.
An OCF dipole is inherently unbalanced, and therefore a voltage balun tends to force common-mode current onto the coax feedline. You might try a 4:1 Current balun at the feedpoint instead of the voltage balun, or simply try one or more chokes on the feedline at various positions between the feedpoint and the shack.
Agreed, with the unbalanced antenna, you need a choke balun to keep the RF out of the shack/house.
As for the TV, you are probably overloading the IF stage in the TV, not interfering with the actual 150+MHZ broadcast signal.
"Ham Radio does not abide by the 'Right Tool for the Job' rule. Quite the contrary.
It often gets the job done with things you wouldnt normally recognize as tools."
Originally Posted by CT2FZI
You need to be cautious about some of the 4:1 Current balun designs you see. Just remember that it's not possible to wind an effective 4:1 Current balun on a single toroidal core - it has to be wound on 2 separate cores. Eplanation here:
Thanks for all your assistance so far.
I will bear what you have said in mind WRT common mode current on the feedline. I have some toroids around somewhere, adding a few can't hurt. We did try adding a toroid to the TV antenna line near the TV end to no effect, but will try some on the radio antenna line.
AC0GR if we simply are overloading the IF of the TV, besides physical separation of the antennas, is there anything else we can do to stop the interference on the TV? IE: would a high pass filter with 42db attenuation for 55Mhz and below be a possible solution?
Regards from down under
Try winding the AC power cord around a torroid. I had the same problem years ago and this is what cured it. Use the proper mix for HF freqs.
Sorry for what is probably a stupid question, but what do you mean by proper mix?
Originally Posted by WE6C
No trees were harmed to bring this post to you.
However, a great many electrons were severely inconvenienced.
#75 or J material are "proper mixes" for HF chokes on AC lines because of the high perms. The commonly available #77 material is also pretty good and is what I use.
Originally Posted by VK2LBP
73, Cecil, www.w5dxp.com
Random length "tuned feeders" usually de-tune
an antenna system (thus requiring a tuner).
When most amateurs talk about toroids, they mean a donut shaped device made of either iron powder, or ferrite.
Either come in many different alloys, powder grain size, manufacturing process, etc.
Depending on the particulars of the above, the resultant toroid (which is a SHAPE, NOT a material, the toroid is sometimes even Teflon, or phenolic, or even air!) will have certain frequency dependant properties.
Here is a link to the Amidon website, There are a few charts that graph the various RF parameters vs. material type.
Using the wrong "mix" can not only give you a device that does not do the intended job, but you can actually end up with a device that makes the problem worse. You may end up with a resonance instead of an absorption or choke.
EDIT: We sometimes use a inductor wound into a toroidal shape because the toroidal shape is Self Shielding, by bringing the + and - (or North and south pole) ends of the coil together, the leakage flux tends to self cancel compared to a equal inductance wound on a rod(solenoid) form.
Last edited by AI3V; 01-15-2010 at 05:17 PM.
Now my mistakes travel at the speed of light!