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Author Topic: Home Brewed Broadband Butterfly Terminated Dipole  (Read 304 times)

N8RFY

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Home Brewed Broadband Butterfly Terminated Dipole
« on: August 25, 2022, 07:14:25 PM »

Hello,

I recently built a Broadband Butterfly Terminated Dipole (BBTD) antenna. Please see the illustration in the link below. I am planning to mount it in my attic in the configuration show in the illustration, except it will be inside of my roof. I was just wondering if it is safe to let attic insulation touch the antenna. I will only be using 100 watts maximum.

https://groups.io/g/hflink/message/6

If I've posted this in the wrong area, please accept my apologies and direct me to the right area for a post of this kind. Any information shared, will be appreciated!


Thanks and 73's,
Calvin
N8RFY
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K6BRN

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #1 on: August 25, 2022, 07:32:04 PM »

Hi Calvin (N8RFY):

The antenna you've selected looks pretty inefficient.  The 1KOhm terminating resistor in the center of the wire  is annotated:  "...power rating in Watts should be the same as transmitter power output in Watts...".  This is a bit of a clue as to where a LOT of the TX power is going - into heating the resistor rather than radiating.  But I'll bet it really helps maintain the match, along with the 16:1 transformer.

Regardless, having fun experimenting and learning is good, and an antenna does not have to be particularly efficient to be useful.  Please let us know how everything works out!

I recently had to do an attic (desperation) install at an HOA, where the unit had a high, peaked roof.  I installed a 63-foot EFHW wire (49:1 transformer) and added a hefty CMC, feeding it with 100 feet of ABR industries LMR-240/RG-8x - pretty lossy coax.  The wire is arranged in a tilted but mostly hoirizontal V.

To my major surprise, this arrangement is working very well on 10/15/20/40M, and even on 17M and 30M where the antenna is non-resonant (coax loss actually helps, here, with match), though it's pretty inefficient.  No RFI at all, either, and noise level is low - very surprising

This is NOT what I expected.  Of course, propagation conditions have been largely excellent. 

Sometimes you get lucky.

Brian - K6BRN
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N8RFY

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #2 on: August 25, 2022, 07:57:13 PM »

Thanks Brian! I will let the forum know how this antenna works out. Did your attic antenna touch, or lay on, any of the insulation in your attic?
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WB6BYU

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #3 on: August 25, 2022, 08:43:48 PM »

Most attic insulation is fiberglass or some other non-conductor.
Letting the wire touch that shouldn't be a problem.

Some insulation has foil backing - that could be an issue with
bare wire, or if the wire is too close to it for too long of a
distance.

I've never had problems with attic antennas arcing to other
materials at 100W, and I've taken a lot of liberties over the
years, but that doesn't mean that you should be careless.

The voltages on a terminated loop shouldn't be as high as
on the ends of a dipole.  I wouldn't expect any arcing to
nearby materials, even wood framing, at 100W, although
some cheap plastic electric fence insulators aren't a bad
idea for maintaining spacing if you have concerns. 
Otherwise, a nail in a board and a short piece of nylon string
are quite adequate for an insulator, and can help to hold it
away from insulation, wood framing, etc.


I've been using similar loops without a termination resistor
for many years, with very good results.  The terminated
ones will be similar to a terminated folded dipole, where
the performance drops (although the SWR improves) at
frequencies below that where the antenna contains about
1 wavelength of wire.  But if 25% to 50% efficiency is
adequate for your application, then it certainly is worth
trying, with the advantage that it won't require repeated
trips to the attic to tune it to resonance.

N8RFY

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #4 on: August 25, 2022, 08:47:26 PM »

Thanks, much appreciated!
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K6BRN

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #5 on: August 25, 2022, 09:17:51 PM »

Hi Calvin (N8RFY):

Thanks Brian! I will let the forum know how this antenna works out. Did your attic antenna touch, or lay on, any of the insulation in your attic?

I suspended my attic wire antenna from eye hooks using non-conductive paracord.  It does not touch the structure, insulation or wiring at any point and the wire itself is insulated.

The antenna works pretty well.  In less than two weeks of casual operation I made over 500 FT8 contacts on 10/15/17/20/30/40M.  >150 on one day alone.  I was running from 5 to 400 Watts.

I'm most surprised that there is no RFI and that the noise level is low.  That's NOT the norm for an attic antenna.

As I said - sometimes you just get lucky.

Brian - K6BRN
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KE0VT

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #6 on: August 25, 2022, 11:18:32 PM »

Calvin, I have a BBTD, 110' long in my attic. You will get about 25W out of 100W signal. My receive is very good. Signal reports usually around
5/4 or 5/5 but sometimes 5/9 and once 20/9. SWR is 1.6/1 or better across all bands from 10-160. 6 meters is 2.35/1 with my TS590SG handling the
tuning very nicely on 6.

With the 1KW resistor heating is minimal, may get warm to the touch. I have worked stations 5700 miles (Japan), Italy, Spain and Slovenia and
Brazil with the Europeans about 5500 miles and the Brazilian over 3000.

I am getting ready to put up a flagpole antenna so hopefully will be able to work more DX. Enjoy your BBTD, just be prepared to work a little harder
for the contacts. Look for me on 17M for a chat.

73, KE0VT
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KF4ZGZ

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #7 on: August 26, 2022, 03:12:56 AM »

If you have a tuner ......
Just for grins and giggles, remove the resistor and make it a solid loop.
Work the same people like this as you did with the resistor and see what they say about your signal.
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Matt

KF4ZGZ

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #8 on: August 26, 2022, 03:13:12 AM »

If you have a tuner ......
Just for grins and giggles, remove the resistor and make it a solid loop.
Work the same people like this as you did with the resistor and see what they say about your signal.
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Matt

WA3SKN

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #9 on: August 26, 2022, 06:16:59 AM »

If the insulation is strictly fiberglass with no aluminum backing, it can touch the antenna.
If the insulation HAS an aluminum backing (for IR purposes) then you probably DO NOT want it to touch the antenna. 

-Mike.
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N8RFY

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Re: Home Brewed Broadband Butterfly Terminated Dipole
« Reply #10 on: August 26, 2022, 07:45:57 AM »

I just wanted to thank everyone for the information shared; very helpful!!!
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