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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Tue 14, 2021 1:44 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17016
Location: S. Dartmouth MA 02748-1225 USA
Troy,

Need to look at the construction of the home, the location of the radio in regard to any outside antenna access and how good or bad and far away the ground (Earth) connection is.

The home construction will determine how well MW and HF will penetrate the walls to reach any loop. What does not work is: Concrete, stucco, brick, stone, aluminum or other metal siding, foil vapor barrier. Such a home is a challenge for loop however, placing the loop near a window facing the direction of interest would help

If an outdoor antenna is chosen if it uses a ground that must be effective, the antenna must also be away from power lines as much as possible. There will be some sort of transmission line for an outdoor antenna. Any coax line must keep its match through all the wavelengths intended to receive or the coax will attenuate the signals when mismatched.

Generally, the larger the loop "aperture" the more selective and directive. The further apart the vertical sides are the more directive, the more spacing of the windings the greater the "Q", all these things can have diminishing returns when the loop becomes very large. Ferrite can be used to load a loop, IMHO that is a field of study on its own, often done experimentally. Any loop that is tuned either remotely or at the loop is that tuned antenna that is desired.

A simple "loosely" twisted pair of #18 wire is a reasonably good low loss transmission line from some six or more feet away. For some antennas avoids the loss from receiving baluns and the coax loss. The twisted pair will receive "some" noise but largely the noise is cancelled by the twist. Twisted pair was favorite of radio fans in the 30's and 40's using such antennas as the delta match and the "spider-web" a multi-band form of the delta match. Orienting this antenna such that it favors the station of interest is practical.

It is also possible to build a weatherproof, outdoor rotatable loop. The outdoor loop would have to be amplified to overcome losses of the transmission line and its baluns. It may get complex if desired to cover both MW and HF...

GL

Chas

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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Tue 14, 2021 4:30 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 809
I asked ashar farhan the famous ham radio wizard if active antenna +BPF would work for MW or not? His reply was he can't say unless it's built. I told him all the experienced engineers are against this idea haha. The thread starter needs to try it!


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Tue 14, 2021 4:49 am 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 804
Location: Colorado Springs, CO 80917
Good stuff, there, Chas! :-)


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Tue 14, 2021 4:55 am 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 804
Location: Colorado Springs, CO 80917
Quote:
I asked ashar farhan the famous ham radio wizard if active antenna +BPF would work for MW or not?

There's no question that "it will work" in some fashion. The relevant questions are:
1. Will it solve the problem in this particular person's situation?
2. Is there a better way to do it than filtering after the amplifying?

I think the answers are:
1. Don't know that for certain.
2. Yes.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Tue 14, 2021 6:48 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 809
An active antenna for 1380KHz.

Front end is tuned plus there's a 5 pole robust BPF between active antenna and receiver. This is the best solution I could come up with under 20 minutes. It will give an acceptable performance. Not much extra noise if it's mounted high outdoors and some distance from the house with a coax delivering 12V and feeding the radio at the same time.

C2 is for impedance match and C1 is to resonate the 22uH coil on 1380KHz.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Tue 14, 2021 6:56 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 809
RA0SMS vs PA0RDT Active Mini Whip - RA0SMS is Very Noisy - Why?
I have been using a PA0RDT active mini whip from Roelof Bakker for about a year now. The PA0RDT antenna has served me well. Clean and able to pull in signals that a 260 foot loop and various long wires cannot.

https://www.sdrplay.com/community/viewtopic.php?t=4128


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Tue 14, 2021 4:46 pm 
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Joined: Sep Wed 03, 2014 11:06 pm
Posts: 328
Location: Poulsbo, Wa
I had problems receiving some distant stations i wanted to hear (lol KRKO was not one - although i do like their format), i read a lot and asked questions but the best advice i got was...
Antennas are like children - they are all different in every situation. ie you just have to try some. I tried dipoles of varying length, long wires of varying length, different size loops, and that EWE antenna i refereed to earlier. For me, here in poulsbo, for the stations i wanted to hear ( KBRC in mt vernon , KBAI in bellingham as well as KRKO -- i like classic rock) the EWE worked best for me. Yes the last antenna i tried.

You just have to experiment until you get what works for you.

Antennas are cheap and easy to build (most of them), it just takes time.

Bill

Ps i do have a spare room for hockey season.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Thu 16, 2021 5:46 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 809
Dare4444 wrote:
An active antenna for 1380KHz.

Front end is tuned plus there's a 5 pole robust BPF between active antenna and receiver. This is the best solution I could come up with under 20 minutes. It will give an acceptable performance. Not much extra noise if it's mounted high outdoors and some distance from the house with a coax delivering 12V and feeding the radio at the same time.

C2 is for impedance match and C1 is to resonate the 22uH coil on 1380KHz.


Any opinions on this?


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Thu 16, 2021 6:52 pm 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 804
Location: Colorado Springs, CO 80917
Quote:
Any opinions on this?

Since you asked...
Active antennas amplify the existing local signals & noise. If a location has a low noise but simply a weak signal level, an small antenna + tuned amplifier might work because the S/N ratio will be decent. A larger or directional antenna might work just as well, too.

If a location has high noise (like most of our neighborhoods do now) and weak signal then a small antenna + tuned amplifier may be useful if the listener is okay with enduring the noise just to hear the signal (e.g., a serious DXer). For comfortable listening, that location needs to raise the S/N ratio, not just amplify the existing S/N; that generally points toward a better antenna &/or antenna location. Once the antenna situation has been optimized, then a tuned amplifier addition may be the only option left (except for settling for listening to the station streaming online).

I'm not sure that an extremely tight filter following a properly designed & operating tuned amplifier has much benefit, unless there's a closely spaced station creating interference with the desired one. A really tight filter adds signal loss, negating some of the gain achieved with the antenna & amplifier.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Fri 17, 2021 8:11 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 809
hwhall wrote:
Quote:
Any opinions on this?

Since you asked...
Active antennas amplify the existing local signals & noise. If a location has a low noise but simply a weak signal level, an small antenna + tuned amplifier might work because the S/N ratio will be decent. A larger or directional antenna might work just as well, too.

If a location has high noise (like most of our neighborhoods do now) and weak signal then a small antenna + tuned amplifier may be useful if the listener is okay with enduring the noise just to hear the signal (e.g., a serious DXer). For comfortable listening, that location needs to raise the S/N ratio, not just amplify the existing S/N; that generally points toward a better antenna &/or antenna location. Once the antenna situation has been optimized, then a tuned amplifier addition may be the only option left (except for settling for listening to the station streaming online).

I'm not sure that an extremely tight filter following a properly designed & operating tuned amplifier has much benefit, unless there's a closely spaced station creating interference with the desired one. A really tight filter adds signal loss, negating some of the gain achieved with the antenna & amplifier.


Thank you! A very precise answer on how the active antenna works and the importance of S/N ratio which is everything. Thank you for giving an indepth explanation. I learned something new! Yes, a tight bpf might induce upto 5dB of insertion loss, a 12dB 2n3866 feedback amplifier after the BPF should terminate it properly, and preserve S/N ratio. Outout BPF is just for super filtration so only the required 1380KHz signal reaches the receiver. No junk.


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