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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 12:21 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1563
soloapollo wrote:
Hello-


But I also wondered if the fact that these are all designed to cover the whole broadcast band might limit how well they work for each particular frequency. It begs the question: can an antenna be designed and built that would optimize reception of a single frequency?
Troy
Edmonds, WA


Yes, if you want to improve your signal to noise ratio it is important that the antenna itself its optimized and resonating at the frequency of the station you want to receive. But there is more:

On medium wave frequencies, these days there is all sorts of electrical noise, from digital apparatus, industrial machinery, line power wiring etc. This makes a wire or whip antenna, even tuned a poor option.

Yet you have been presented with non tuned whip antennas feeding broad band RF amplifiers, which is exactly what you do not want unless you expect to hear your radio station drowned out by every pop crackle, splat and buzz from the electrical interference, which likely may include apparatus in your own home, like, TV's, computers, CFL lamps , solar inverter installations, induction cook tops, switch-mode psu's and even your home's security system that sends digital fast rise signals down many meters of wire to control panels... where should I stop.

What you need is a tuned ferrite rod antenna which responds to the magnetic component of the field, tuned to the frequency of your far off radio station, and rotated for maximum pickup. Then I would recommend a 3 to 4 turn coil added to that (or use the one that is normally there on a transistor radio ferrite rod coil tuned by the usual polyvaricon or miniature variable capacitor), feed that coil into the antenna input of your radio. If that is not satisfactory, at that point you could add a single tuned (at the station frequency) transistor RF stage, which would elevate the gain at the signal frequency, but not so much for the noise voltages in the region above and below your station frequency.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 12:29 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 732
ACORNVALVE wrote:
soloapollo wrote:
Hello-


But I also wondered if the fact that these are all designed to cover the whole broadcast band might limit how well they work for each particular frequency. It begs the question: can an antenna be designed and built that would optimize reception of a single frequency?
Troy
Edmonds, WA


Yes, if you want to improve your signal to noise ratio it is important that the antenna itself its optimized and resonating at the frequency of the station you want to receive. But there is more:

On medium wave frequencies, these days there is all sorts of electrical noise, from digital apparatus, industrial machinery, line power wiring etc. This makes a wire or whip antenna, even tuned a poor option.

Yet you have been presented with non tuned whip antennas feeding broad band RF amplifiers, which is exactly what you do not want unless you expect to hear your radio station drowned out by every pop crackle, splat and buzz from the electrical interference, which likely may include apparatus in you own home, like, TV's, computers, CFL lamps , solar inverter installations, induction cook tops, switch-mode psu's and even your home's security system that sends digital fast rise signals down many meters of wire to control panels... where should I stop.

What you need is a tuned ferrite rod antenna, tuned to the frequency of your far off radio station, and rotated for maximum pickup. Then I would recommend a 3 to 4 turn coil added to that (or use the one that is normally there on a transistor radio ferrite rod coil tuned by the usual polyvaricon or miniature variable capacitor), feed that coil into the antenna input of your radio. If that is not satisfactory, at that point you could add a single tuned (at the station frequency) transistor RF stage, which would elevate the gain at the signal frequency, but not so much for the noise voltages in the region above and below your station frequency.


Excellent! A tuned ferrite coil with a tap at ground end for low impedance out to low noise 2n4401 preamp. Is the mini whip useless for medium wave band? Mini whip is suppose to be installed outside the house.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 12:44 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 732
The actual antenna element is not a metal plate, as usual in MiniWhip antennas, but a bent thick copper wire. Since most of the charge on a metal plate sits at the edges (see here), it still works almost as well if only the edges are there, in the form of this wire. The wire looks like a coil of two turns, but it isn't: at the bottom, all wires are connected.

The mini whip seems to work well at MW as per the reviews.

http://www.pa3fwm.nl/projects/miniwhip/


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Last edited by Dare4444 on Sep Sun 12, 2021 12:49 pm, edited 2 times in total.
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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 12:45 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1563
Dare4444 wrote:

Excellent! A tuned ferrite coil with a tap at ground end for low impedance out to low noise 2n4401 preamp. Is the mini whip useless for medium wave band? Mini whip is suppose to be installed outside the house.


You can resonate a mini whip or at least 4 to 6 feet rods, at MW band frequencies, it tends to work better if you have a reasonably large ground plane like a car's body. In MW Car radios it was successfully achieved to resonate the car's whip antenna (including the capacitance of the coax feed) at around 1400kHz, it is more rarely achieved with your average portable MW radio with a whip, which is generally nearly always why they have a ferrite rod. If you look up vintage car radios, and look at the antenna input circuits, you will see how they did it.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 4:03 pm 
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Location: San Antonio, TX
I'll bet the OP has a headache this morning :(

Dale


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 4:42 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10930
Location: Santa Rosa, CA
I wouldn't waste any time on an active device or active antennas. You have to capture a decent signal before you amplify it.

Folks here already gave a workable solution....a large (23" or larger) directional loop. It will tune very sharply to one frequency and you can orient it to null undesired stations.

I built a hula hoop loop (about 36" diameter) for a friend who wanted to listen to the SF Giants games from his "cabin" in Mendocino, about 150 miles away. I was able to get reasonable reception of Seattle AM from inside a steel-framed building. Loops are simple, cheap, and effective. Larger is better.

http://kr1s.kearman.com/html/hooploop.html

I didn't build any matching network, just put a small radio inside the loop. The loop couples to the radio's internal ferrite bar antenna.

https://www.instructables.com/Medium-Wave-AM-broadcast-band-resonant-loop-antenn/

Here is a loop I built for shortwave:

Image

Rich


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 6:47 pm 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 704
Location: Colorado Springs, CO 80917
Think of this problem this way & see if it doesn't make sense:

When you insert an amplifier (active antenna) onto the input of your existing radio, you are actually modifying the radio by adding an additional RF stage (or 2 or 3) at the front end. How often do you see practical radio designs with wideband untuned RF amplifiers at the front end?

Toss in the reality that most AM broadcast receivers have just one RF "tank" circuit before the mixer, the radio could REALLY use better selectivity up front to accompany a greatly increased input level from another RF amp at the front end.

Those untuned antenna amplifiers accept signals & noise at ALL frequencies. Local or strong signals or noise AT ANY FREQUENCY can drive the amplifier into nonlinearity, with all kinds of distortion and additional RF crud being generated.

ACORNVALVE & Rich, W3HWJ have the right idea, I think. The OP probably wants more S/N ratio, not just an amplifier increasing the signal level + all the noise.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 9:36 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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hwhall wrote:
Think of this problem this way & see if it doesn't make sense:

When you insert an amplifier (active antenna) onto the input of your existing radio, you are actually modifying the radio by adding an additional RF stage (or 2 or 3) at the front end. How often do you see practical radio designs with wideband untuned RF amplifiers at the front end?

Toss in the reality that most AM broadcast receivers have just one RF "tank" circuit before the mixer, the radio could REALLY use better selectivity up front to accompany a greatly increased input level from another RF amp at the front end.

Those untuned antenna amplifiers accept signals & noise at ALL frequencies. Local or strong signals or noise AT ANY FREQUENCY can drive the amplifier into nonlinearity, with all kinds of distortion and additional RF crud being generated.

ACORNVALVE & Rich, W3HWJ have the right idea, I think. The OP probably wants more S/N ratio, not just an amplifier increasing the signal level + all the noise.


I did mention a narrow BPF centered at 1380KHz , and it's a 5 pole. Wouldn't it remove all other signals? Bw is just 40KHz. High attenuation of all other frequencies except 1360 to 1400 KHz. A loop antenna is directional but the signal is all over the place in varying field intensity. The copper plate in the active whip will effectively tap into it and amplify. Then the matching BPF removes all other junk and extract his pure 1380KHz signal. Think about it. It can be built with junkbox parts. 4.7uH axial chokes are readymade parts sold online.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 10:14 pm 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 704
Location: Colorado Springs, CO 80917
Quote:
I did mention a narrow BPF centered at 1380KHz , and it's a 5 pole. Wouldn't it remove all other signals?

The bandpass filter is in the wrong place, _after_ the amplifier. My point is that the amplifier ought to have some selectivity in front of it.

The filter will indeed only pass a narrow band of signals but they will be whatever noise + signals the amplifier coughed out, which may include intermod, etc. It's better to feed the amp only what you want amplified.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 10:28 pm 
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Joined: Mar Thu 01, 2018 1:30 am
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hwhall wrote:
Quote:
I did mention a narrow BPF centered at 1380KHz , and it's a 5 pole. Wouldn't it remove all other signals?

The bandpass filter is in the wrong place, _after_ the amplifier. My point is that the amplifier ought to have some selectivity in front of it.

The filter will indeed only pass a narrow band of signals but they will be whatever noise + signals the amplifier coughed out, which may include intermod, etc. It's better to feed the amp only what you want amplified.


It's tricky. The input to active whip is high impedance. But I've read good reviews that it works fine for MW and SW even with its wideband gain.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 10:31 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 732
Medium wave DXing used to be a hobby in its own right. I’d spend hours tuning up and down the band, listening to stations from around the world…. But noise began to creep in and finally destroyed the band. However, something happened to bring medium wave back to life – the Mini-Whip Active Antenna arrived. The results are unbelievable! With little or no noise wiping out the band, medium wave is packed with stations!


https://www.radio-workshop.co.uk/g4nsj- ... ing-am-dx/

Interesting read!


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Sun 12, 2021 10:42 pm 
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Location: S. Dartmouth MA 02748-1225 USA
hwhall wrote:
Quote:
I did mention a narrow BPF centered at 1380KHz , and it's a 5 pole. Wouldn't it remove all other signals?

The bandpass filter is in the wrong place, _after_ the amplifier. My point is that the amplifier ought to have some selectivity in front of it.

The filter will indeed only pass a narrow band of signals but they will be whatever noise + signals the amplifier coughed out, which may include intermod, etc. It's better to feed the amp only what you want amplified.
+1

Chas

_________________
List' & I will Enchant Thine Ear


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Mon 13, 2021 1:25 am 
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Joined: Mar Thu 01, 2018 1:30 am
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I bet everyone denying the active antenna here never used one for the MW band. The internet is filled with comments praising the active whip for medium wave band. Don't dismiss it without first trying it out. The active antenna with copper plate is readily available on the internet. The BPF I mentioned is the only home brewing that needs to be done. Why so much mention of Intermodulation distortion gonna wipe out MW signal with noise? How are you guys so sure? We are just confusing the original poster with our assumptions. Remember that perfect is the enemy of good. The 40m band at night is filled with powerful AM stations generating Intermodulation and yet the active antenna with the copper plate shines! The active antenna directly feeds the band pass filter in the radio's front end. This is exactly what I proposed. The person who started this thread can buy the mini whip original having the copper plate on the internet and build the BPF on a copper clad board along with some help if necessary and he's done. Mount the active antenna outdoors on a 30 foot mast and bam he would be picking up distant medium wave stations just fine! The loop antenna and stuff is too complicated for a person having little experience in electronics.

Watch "G4NSJ - Mini-Whip active antenna on long and medium wave" on YouTube
https://youtu.be/bvgUFWpoYuc

Watch "Mini Whip Active Antenna - AM/MW DX of News Radio 1180 WHAM @ 1180 kHz from Rochester, New York" on YouTube
https://youtu.be/Ryco8mia0RQ

Wire antenna matching >> Watch "Improving AM Radio Reception w/ External Antenna and Loop Connection" on YouTube
https://youtu.be/d1jaojiA1Oo


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Mon 13, 2021 1:54 am 
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https://www.google.com/url?sa=t&source= ... phUT8riFNz

The original active whip PDF. See the attached screenshot. Bandwidth narrowed down to LW and MW.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Mon 13, 2021 2:11 am 
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ACTIVE ANTENNA - PA0RDT - Version 2
PA0RDT compact active wide-band receiving antenna for lower frequency bands from 10 kHz to approximately 10MHz.

https://vk6ysf.com/active_antenna_v2.htm

Just make this and add the BPF between Ant and Rx. It's the simplest solution. Nothing much to it really. I gave the simplest and effective solution. Now done.

.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Mon 13, 2021 2:57 am 
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Joined: Jun Sun 15, 2014 5:37 pm
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Location: Montreal, Quebec
This is getting crazy.

An active antenna as described almost fifty years ago was a short antenna directly coupled to a very high impedance point. The goal wasn't primarily amplification, but impedance step down. At least some paid attention to strong signal handling, after all, it sees everything that arrives at the antenna.

Like I said, it's not unlike tube car radios. A short antenna directly coupled to the top of a tuned circuit, ie a high impedance point. But there, the tuned circuit limits what the active stage sees.

You see a fusion in some shortwave portables, the whip feeding the gate of a jfet, then the standard front end with tuned circuits.

But the concept has been garbled, an untuned amplifier connected to a whip. But these schematics disconnected from text, disconnected from the original concept, ignore the fact that preamps in the old days really didn't do much, unless they included tuned circuits.

You have to understand the problem before throwing solutions.

Is the issue gain, selectivity, or an antenna that captures more signal?

Putting a filter after gain won't do a thing, unless the amplifier can handle tremendous signal levels. And then why the filter? Putting the filter, even a tuned circuit first, means the amplifier doesn't have to handle so.much signal.

But putting a random filter between a whip antenna and a real active antenna won't work, it all relies on a very high impedance connected to the whip.

Just about everything already exists, no need for simulation, but certainly a need for a broad understanding to look at circuits and decide their value.

A tuned loop, again, has the advantage of providing some selectivity, and directionality, two key components when DXing.

Fifty years ago there were lots of projects of a loopstick antenna into a preamp. Yiu get some selectivity, gain, and directionality. The one thing it doesn't do is get the antenna out of the house full of noise generating devices.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Mon 13, 2021 3:26 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 732
J310 has very good signal handling capability and the next stage the 2n3866 is biased for 40ma of current and runs hot. These are well tested designs used by hundreds and thousands of ham radio operators. 2n3866 @ 40ma approaches 1dB compression point of 21dBm or 100mW!

I am thinking of designing an active antenna with IRF510 front end with large standing current or with parallel BS170.

I've no doubt the active whip feeding a 5 pole and narrow BPF will perform flawlessly on the MW band. 21dBm of compression point is a respectable figure plus the IP3 is +33dBm which is high and offers a large dynamic range! Mount the mini whip outside and away from the house. A 30 feet mast would do wonders.

Where is the problem here? Of course one needs to test its performance. I haven't but hams love it.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Mon 13, 2021 4:52 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10930
Location: Santa Rosa, CA
Quote:
I bet everyone denying the active antenna here never used one for the MW band. The internet is filled with comments praising the active whip for medium wave band. Don't dismiss it without first trying it out.


Same goes for a 23 inch MW tuned loop. Try it, you will like it! No SPICE needed.

Rich

PS: I actually don't believe everything I read on the Internet.

PPS: I used to work for Siliconix. Good luck with the IRF510. If you have some vintage pieces, they may conform to the datasheet. Nowadays, old-time devices like that are made somewhere in Asia and it's not certain what kind of chip is used. It was designed as a power switching transistor when our technology was pretty primitive. Lots of them disbursed into the surplus market, so they seem to get used for everything. RF parameters, like capacitance, were never tested in production... they were"typical."


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Mon 13, 2021 6:48 pm 
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Joined: Jan Sat 25, 2020 5:23 am
Posts: 704
Location: Colorado Springs, CO 80917
I'm going to toss in one more potential problem with the untuned amplifiers between the antenna & the stock radio.

Ordinary consumer BCB radios have almost always been skimpy in the front end selectivity. Often just one tuned circuit before the mixer. Consequently, stronger stations will sometimes appear more than once when you tune across the band; the proper station position on the dial plus "images" with lower signal strengths.

Example: BCB sets all use high-side injection, so if I tune for a station at 650kc, the local osc will be at 1105kc & a strong station at 1560kc can also be appear as low-side injection & attenuated only by the single tuned circuit.

Now, then, when you increase the signal strength of ALL the stations all across the band, the likelihood of images being loud enough to be noticed increases, leading to more chances of interference. It's also possible that is a reason it was remarked that with those amplifiers the BCB band appeared to be "packed" - some stations may have been appearing more than once as images.

Anyhooo... more selectivity up front is seldom a bad thing & combined with amplification it simply becomes an ever better idea.


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 Post subject: Re: How to build an AM antenna optimized for a single freque
PostPosted: Sep Tue 14, 2021 12:06 am 
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Joined: Jun Fri 26, 2020 9:35 pm
Posts: 74
Location: 98020
Hello-

Wow! Thank you for all the responses. Here's some answers to a couple of comments/questions that were asked.

Yes, the station is KRKO 1380. "Flamethrower" may describe the signal across the water in Poulsbo, but here in my corner of Edmonds, it is rather more like "anemic peashooter". This is not always the case; it's primarily after dark and during the winter weather that the signal can be unreliable. Naturally, this corresponds to peak hockey season when I most want the best reception.

No, I don't have a headache from all of this, but there is definitely quite a lot to think about and figure out how best to build and implement. It may take me a while to get an opportunity to try these ideas out.

The radio I am using is an E.H. Scott Allwave 23 from late 1936 (7 knob version), so I think it is on the good side of radio design from that era.

Although I cannot eliminate all sources of present-day interference from inside the house, my own devices that generate such noise are at a minimum; certainly I have fewer of them than a typical home today.

I'll have to go through all of these ideas more thoroughly to really understand what all of them are.

I appreciate all the responses and suggestions!

Troy
Edmonds, WA


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