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Post by Admin on Nov 27, 2017 21:27:52 GMT
In anticipation of someday putting my transmitter online at 13.560 mHz, I occasionally check to see if there is activity.
Today I'm hearing a low frequency buzz centered on the frequency. There was a quick CW ID and then the signal stopped.
It didn't seem to fade in/out like skywave and the signal was not weak.
I should listen more often.
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Post by Druid Hills Radio on Nov 28, 2017 13:24:35 GMT
In anticipation of someday putting my transmitter online at 13.560 mHz, I occasionally check to see if there is activity. Today I'm hearing a low frequency buzz centered on the frequency. There was a quick CW ID and then the signal stopped. It didn't seem to fade in/out like skywave and the signal was not weak. I should listen more often. Let us know when your signal is on. 13.560 would typically have a limited local range but even at low power with say a dipole antenna, the right conditions your signal could be heard thousands of miles away. |
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Post by Deleted on Nov 28, 2017 14:28:09 GMT
Short Wave ActionIn the recent past several members of part15_us participated in the design of a shortwave AM transmitter for use at 13.560 MHz under the project name: Big Talker. The prototype built here at Worldround Radio Laboratory is spread out on the workbench for installation into a metal case. Upon completion the transmitter will feed an "End-Fed Dipole" of a type previously decribed by MRAM 1500, horizontally beamed east and west. Perhaps we can be on the air in time for Xmas... Your Big Talker Link
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Post by Druid Hills Radio on Nov 28, 2017 15:35:19 GMT
Short Wave ActionIn the recent past several members of part15_us participated in the design of a shortwave AM transmitter for use at 13.560 MHz under the project name: Big Talker. The prototype built here at Worldround Radio Laboratory is spread out on the workbench for installation into a metal case. Upon completion the transmitter will feed an "End-Fed Dipole" of a type previously decribed by MRAM 1500, horizontally beamed east and west. Perhaps we can be on the air in time for Xmas... Your Big Talker Link There is NO SUCH THING as an end-fed dipole. There is an end-fed half-wave antenna. A dipole by virtue has two elements. You stand corrected. |
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Post by Admin on Nov 30, 2017 21:31:15 GMT
No, actually this was explained some time ago.
It is not an end fed half wave radiator. Rather it is a quarter wave radiator that works against a quarter wave outer coax shield.
One quarter wave from the center a ferite choke is placed on the coax effectively making that point a very high impedance not unlike a ferite balun placed on coax at the feed point of a dipole.
The outer coax shield is then one half of the dipole. The other half is connected to the center conductor.
Think of it like the old "coaxial antenna.". The difference being for that antenna the coax runs thru the bottom section to the middle feed point. The end of the bottom section is an open or very high impedance.
The end fed dipole antenna achieves that "open" at the quarter wave point using the ferite choke. The choke only affects RF current on the outside of the coax.
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Post by Druid Hills Radio on Nov 30, 2017 21:36:34 GMT
No, actually this was explained some time ago. It is not an end fed half wave radiator. Rather it is a quarter wave radiator that works against a quarter wave outer coax shield. One quarter wave from the center a ferite choke is placed on the coax effectively making that point a very high impedance not unlike a ferite balun placed on coax at the feed point of a dipole. The outer coax shield is then one half of the dipole. The other half is connected to the center conductor. Think of it like the old "coaxial antenna.". The difference being for that antenna the coax runs thru the bottom section to the middle feed point. The end of the bottom section is an open or very high impedance. The end fed dipole antenna achieves that "open" at the quarter wave point using the ferite choke. The choke only affects RF current on the outside of the coax. That makes sense. I made such an antenna years ago but it was vertically polarized. I was called a coaxial dipole. |
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Post by Admin on Dec 1, 2017 18:49:38 GMT
Yes, I suppose you could hang it at any angle. It makes a convenient way to put up a horizontal dipole running from the house out to a support like a pole or tree. Being end fed the coax doesn't have to hang from the middle and slope back to the house.
The coaxial dipole was used for CB and other as it's easy to put up, no drooping radials. Again the main difference is that antenna runs the coax up through the lower "pipe" section to the middle feed point where as the end fed dipole achieves the same effect using the choke at the "end" of the antenna to create that hi-Z, open point.
It's recommended for the center fed dipole to place a choke on the coax at the feed point to prevent current flowing down the coax shield which skews the radiation pattern. Although I've never done that and my center fed dipole seems to work okay for me. Perhaps it could work even better with the choke.
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Post by Druid Hills Radio on Dec 1, 2017 20:18:09 GMT
Short Wave ActionIn the recent past several members of part15_us participated in the design of a shortwave AM transmitter for use at 13.560 MHz under the project name: Big Talker. The prototype built here at Worldround Radio Laboratory is spread out on the workbench for installation into a metal case. Upon completion the transmitter will feed an "End-Fed Dipole" of a type previously decribed by MRAM 1500, horizontally beamed east and west. Perhaps we can be on the air in time for Xmas... Your Big Talker Link There is NO SUCH THING as an end-fed dipole. There is an end-fed half-wave antenna. A dipole by virtue has two elements. You stand corrected. I owe Carl an apology. |
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Post by Deleted on Dec 1, 2017 21:19:20 GMT
Holy Smokes
That is the largest apology I've ever received, Druid Hills.
I accept your apology because I'm afraid not to.
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Post by Boomer on Dec 2, 2017 14:41:19 GMT
Wi-Fi access points look like they use the same kind of coaxial dipole, stripping off the end of a piece of coax to a quarter wavelength which is about 3/4" inch or a few cm., then folding the braid back down over the outer shield, and clamping a thick brass or silver colored tube over the braid that's probably a specified length for the lower half of the dipole at that frequency.
It's probably very easy to do by machine, and well, it works on our wifi.
It seems to me that the dipole is fed RF power at the end, but ultimately the feed point is still the center split, the point where radiation really begins.
Boomer
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Post by Admin on Dec 4, 2017 18:04:54 GMT
Yes, folding the shield down over the cable would be like the conventional coaxial antenna described above.
Also end fed but no choke instead using the shield as the "lower pipe" section.
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