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Old 26th Jun 2017, 3:41 pm   #61
Al (astral highway)
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Default Re: Broadband transmission line transformer design ?

Wow! That developed some pace over the weekend! I need to catch up on the unfolding debate in detail!!!
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Old 26th Jun 2017, 10:39 pm   #62
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Default Re: Broadband transmission line transformer design ?

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Originally Posted by G0HZU_JMR View Post
Argus, can I ask what kind of range have you achieved with a fairly modest PA and an antenna designed for some degree of far field radiation efficiency?
I'll give some examples:

With the modified Tek sine wave generator as the source, it puts out exactly 100mW into a 50R load close to 1500kHz, and with the smaller of my two multi-turn loops (both crafted for a near exact 50R input) the small grey one is only 320x320 mm square, the range is good to about 20 to 25 meters. Using the bigger orange loop(about 430x430) its about 35 to 40m. But effects in the near field and obstructions affect it, so that would be direct line of sight. For example with the small grey antenna and 100mW, it transmits though a wall & well to a location outside about 10m to 15m away, not much more for a solid signal. Brief experiments with a different transmitter and the orange antenna, with an exact one watt carrier, it can get well over 50m range and is far too much.

(Anyway on my previous post I think it is much better to build RF amplifiers and match them to resistive loads, then also attempt to make the antenna look like a resistive load at the operating frequency, rather than struggle with unique matching networks for various amps and antennas. Once the amplifier is characterized for its performance into a resistive dummy load, at least you know where you stand with it, its power output and efficiency can be measured, so you can be sure to stay in any legal boundaries too. And its not too difficult to make the antenna look like a resistive load of the same value at its operating frequency. The results you get then depend on the radiation resistance of the antenna, which in this case needs to be a multi-turn loop unless made bigger in size than the ones cited above).
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Old 27th Jun 2017, 12:30 am   #63
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Default Re: Broadband transmission line transformer design ?

Thanks for the info. I didn't realise the dimensions were quite that small for your antennas. Having made an exact replica of Charles' copper pipe antenna it is very bulky at nearly 760mm x 760mm and would have to go in the loft if I were to keep it.

I measured the copper loop on its own on the VNA and got 2.5uH at 1.6MHz. The ESR was about 0.04 ohm but this is not measured in a clear uncluttered space. I made the 8nF caps from 2x 3990pF Suflex caps in parallel (so 4 in total used for the antenna). Each Suflex cap measured 4nF at 1.6MHz and each 4nF cap had an ESR of about 0.056R at 1.6MHz.

When made up into a complete antenna the Rp looking into the tap at resonance was about 1500 ohms and I ended up putting a 12k resistor across the loop to get down to about 1000 ohms at the capacitive tap to suit the modulator and to keep it as per Charles target here. This would mean the Rp across the loop would be 4000 ohms and this agrees pretty well with Charles' figures. With the antenna like this it should give about 10-11kHz bandwidth. So this means just over 5kHz AF BW.

I tested the modulator for BW at 1.6MHz using a flat AF noise source and the modulated transmission at 1.6MHz gave about 5.6kHz AF BW when detected/demodulated with the Racal 9008M and fed to the RSA3408A analyser. See the plot below. It shows the audio BW is -3dB at 5.6kHz. I know Al doesn't want to use the single turn loop but I've included this info for reference in case anyone is interested. The above VNA measurements agree with Charles' data and show the need for the 10k (nominal) resistor. It also shows that the bandwidth is best suited for talk stations

What bandwidth do you get with your antennas? I guess you have a wider BW to suit music transmissions. You may have given this info already somewhere but I missed it.
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Old 27th Jun 2017, 4:39 am   #64
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Default Re: Broadband transmission line transformer design ?

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What bandwidth do you get with your antennas? I guess you have a wider BW to suit music transmissions.
I did not record the value at the time. I would have to sweep it to get the data.

In my audio processor circuit there is no pre-emphasis or de-emphasis of the audio, just volume leveling and soft peak clipping. With the music source material (derived from an iPod) and tuning into the carrier; its overall sound is indistinguishable in audio spectrum from any other AM station for the audio high frequencies, on my Hacker Sovereign (which has better audio quality than most transistor radios, in my opinion).

It is probably not surprising, if you consider a typical superhet radio multi-turn frame antenna; this tuned stage is not what limits the audio bandwidth for the radio, it is fairly broad and the bandwidth limiting factor is in the 455KHz IF stages where most of the gain is. Even with a 3 stage TRF radio the bandwidth is normally still better than a superhet on account of the bandwidth limitations of the typical 3 transformer IF amplifier.

So one might expect a similar response/bandwidth using the tuned frame antenna for transmitter service since receiving and transmitting functions of any antenna are two sides of the same coin, but likely a little broader in response due to the heavier loading in the transmitting case.

I'm unaware of any attenuation of the HF audio for music, beyond any other station. Mind you, my own HF hearing response is lower as I age and I think its down to about 8 to 10KHz now but that is still ok for AM radio.

(One experiment I tried was to find out how far 100mW could transmit when not limited to a compact loop. I constructed a very long tuned vertical antenna with a large artificial ground plane and huge base loading coil and was astonished at the range, I will look for a photo of it and post it, if I can find it)

Last edited by Argus25; 27th Jun 2017 at 4:56 am. Reason: add info.
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Old 27th Jun 2017, 9:14 pm   #65
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Default Re: Broadband transmission line transformer design ?

Quote:
(One experiment I tried was to find out how far 100mW could transmit when not limited to a compact loop. I constructed a very long tuned vertical antenna with a large artificial ground plane and huge base loading coil and was astonished at the range, I will look for a photo of it and post it, if I can find it)
Ahah, yes please, that would be very interesting.

Remember, if you didn't already know, I did this experiment around ten years ago

http://www.vintage-radio.net/forum/s...ad.php?t=24791

This is when I built a converter to listen to moribund carrier waves from the original NASA space effort

..so I have a long history of listening to signals propagated from a weak source, and across huge distances. I'm much more there than in plugging in huge powers and mismatches to get a reasonable result!
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Old 27th Jun 2017, 9:36 pm   #66
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Default Re: Broadband transmission line transformer design ?

When I first got my class A licence I tried transmitting on virtually every HF band within the first few days. This was back in the late 1980s and I looked in my logbook and in 1987 I said a quick hello to the regulars on the AMPS (AM Preservation Society?) net on 1.9MHz/topband.

This was a group of old timers who met up on the band every Sunday afternoon to use AM rather than SSB. They were mainly based in Essex or Kent and I was in Chelmsford at the time. The chairperson of the net was Dave G4GVO and he was in Leigh on Sea, Southend and my signal report to him (20km away) was 5/7. Although signal reports are open to interpretation he said I was a decent clear signal. I was using 8W power into maybe 50 feet of wire via a manual ATU with no proper grounding system. It was a fairly grim antenna for topband but several stations could hear me weakly at further distances across Essex. If I'd gone down to 100mW I think I would have failed to make a contact but I'd have expected it to go more than 2km to get the same 5/7 report if someone was closer to me. Had I used a better antenna system I would have been heard much stronger and could probably have exploited night time propagation to cover a very large distance on just 8W if the conditions were favourable.
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