AM Broadcast dilemma using longwire

[kruser]

Hi all,

I've used a simple end fed long wire for years and I like to DX the AM broadcast band as well as lower for NDB’s. My long wires are end fed with the end going to the center conductor of 30 foot lengths of RG-58 coax. The coax shield is left open outdoors due to living on the top floor of an apartment complex with no way to properly ground it. I use coax to the long wires to try and have a shield from all the indoor man made noise. The indoor end of the coax is grounded both to the radio gear and a confirmed grounded water pipe. The indoor ends are fed into a MFJ-1026 Noise Cancelling device that helps greatly to reduce the local hash that is generated from stuff in others apartments. Without the MFJ, HF monitoring would be useless mostly. I have two long wires setup the same. One is the primary antenna and the other is used as a noise pickup antenna for the MFJ unit to function.
The output from the MFJ is fed to one of the HF versions of a multicoupler from Stridsberg which then feeds my various receivers which are mainly an Yaesu FT-1000 and an Icom R9000. Those two receivers work fantastic with this setup on all bands including AM broadcast and NDB's.

Here is the kicker, I've had an HD radio receiver for some time but have never hooked up an antenna to the AM terminals for the few AM stations here that are in HD. So I ran a coax from a spare output on the Stridsberg coupler to the HD radio and I can hardly hear anything. I did use a 75 to 300 ohm balun from a TV antenna as the HD radio has wire inputs for a small loop antenna which is useless in a RF noisy place like an apartment complex.
I'm pretty sure the Stridsberg unit is 50 ohms as are the Yaesu and Icom inputs.
What is causing this to fail when I hook a coax to the HD radio via a balun? I'm stumped.

I did try the obvious and swapped ports on the Stridsberg as well as try a different coax. No change. The same coax feeds the Icom just fine so that part is all good. Is it the mismatch at the balun that is messing me up? Or do the typical TV antenna 75 to 300 ohm balun’s filter signals below a certain frequency? I did try several different brand matching (balun) transformers with the same horrible results.
I also dug out an old Pioneer stereo receiver I have made back in the 70's that had a super AM receiver section and it also will not work with this antenna setup. Again, I'm stumped and obviously overlooking something simple and stupid. Why does the same antenna connection work so well on my Icom's and Yaesu's but not on the standard home type broadcast band receivers like made by Pioneer and Sansui for example?

I also attempted a direct hookup with an alligator clip lead from the coax directly to the radio receivers in question which takes the matching transformer out of the picture but still the same very poor results.
Do consumer stereo receivers just have sucky AM receiver sections in them compared to gear from Icom or Yaesu?
I'm still thinking it is something with an impedance mismatch but before I go experimenting, I figured I'd ask here as I feel it is something simple I'm overlooking.
I do have variable caps and such and can construct something to bring things back to a closer impedance match if that is what is needed but I'm looking for advice on where to start.

Currently, I can do better by touching a short wire shoved into the antenna connector clips with my finger than I'm getting from my long wires!
My long wires are maybe 150 foot in length and work perfect with the Yaesu or Icom as well as several other amateur type receivers.

Does anyone have any advice? I’m really stumped by this one.

Thanks!

 

[k9rzz]

"Currently, I can do better by touching a short wire shoved into the antenna connector clips with my finger than I'm getting from my long wires!"

Maybe you're just overloading the front end then? Maybe some loose coupling with some small wire loops or small capacitors would work?

 

[kruser]

"Currently, I can do better by touching a short wire shoved into the antenna connector clips with my finger than I'm getting from my long wires!"

Maybe you're just overloading the front end then? Maybe some loose coupling with some small wire loops or small capacitors would work?

I will certainly try that and see. I did think about overload plus I also found an antenna tuner I had stashed away that tunes down into that range. It has baluns built in that allow balanced and unbalanced antennas as well as 50 ohm antennas so I'll try it as well.
I sure don't recall having any issues like this when i was much younger but I did not live in a high RF environment like I do now back in my much younger years. I recall the "wow" factor back in the early years when I plugged in a longwire back then which is the exact opposite from what I get now.

Thanks for the suggestions!

 

[Thayne]

I kinda think you might be onto something with the impedance matching theory. (Just guessing though)

Old radios usually just had an antenna terminal & ground & may by design have been high impedance.

 

[prcguy]

A 75 to 300ohm TV balun is rated from 54MHz and up, at 1MHz it would be something other than a balun and will have very high loss.

Also, a wavelength at the bottom of the AM broadcast band is about 1,700ft and a 1/4 wavelength is about 433ft. Your long wire antennas are actually very short for the AM broadcast band and probably present a very high impedance to the 50ohm coax, causing a big loss in voltage (signal) at the feedpoint.
prcguy

 

[kruser]

A 75 to 300ohm TV balun is rated from 54MHz and up, at 1MHz it would be something other than a balun and will have very high loss.

Also, a wavelength at the bottom of the AM broadcast band is about 1,700ft and a 1/4 wavelength is about 433ft. Your long wire antennas are actually very short for the AM broadcast band and probably present a very high impedance to the 50ohm coax, causing a big loss in voltage (signal) at the feedpoint.
prcguy

Yep, I did take the TV balun out of the picture and had some better results but then I realized my embarrassing mistake. I had reversed the antenna inputs on the rear of the old Pioneer receiver. I was hooking the feed to ground. I blame it on old age and poor eyesight.

Anyway, that helped plus I played around with some variable tuning caps and made more progress. The thing that puzzles me though is why the amateur radio receivers and the Icom R9000 work so well even with my short wire yet the old receivers from the 70's and 80's have such a hard time with it. I never expected them to work perfect but at the same time, I was shocked at how poor they did work. Even after I corrected my simple mistake they are still poor.
I have learned one definite thing. The noise generated by all the modern day stuff that is found in a guy’s apartment that is full of tons of electronics is amazing. I’ve spent countless hours finding and curing local noise but mainly for 25 MHz and higher. Now it looks like I’ll spend the same hours working on the stuff below 25 MHz. It seems the old AM/FM receivers are more susceptible to this noise compared to the Icom's and Yaesu's even though the old audio receivers are built like tanks with heavy gauge shielded enclosures.
I've been playing with a loop I built today and it works pretty well on the old home receivers and the modern HD receiver. The longwire still beats it on the Icom or Yaesu though but I think it is just a matter of finding the right parts to get a matched signal into the receiver. I had to get the loop outdoors though as the local noise killed any chance at getting a signal. I can actually use the loop now to null the noise.
I also toyed around with loosely coupling the signal lead to see if it was an overload problem. I think I'm OK on that front.

Thanks for the suggestions everyone. Things are working much better now and I'm still making progress.

edit: One other thing - The HD receiver is just a cheap thing made by Insignia that I picked up at a local Goodwill store for $5 some time back. I never expected much from it but it did end up working ok on the FM band. Today while messing around with the loop I built, I discovered that the Insiginia is its own enemy in the AM band. This thing generates so much noise internally that I'm surprised it worked at all. Luckily it has an optical output so that lead is not radiating anything outside the box but the power cord did.
I was able to clean it up fairly well by installing ferrite chokes on two flat ribbons that connect the front panel and display to the main board plus I added two large ferrites on the power cord. Those greatly reduced some noise I found that was radiating back into the Icom.
I really expected to find a switching power supply inside but it does have a linear supply. I think that is about the only good thing about it!

 

[majoco]

Make yer own transformer for 450 to 50 ohms.......

http://www.dxing.info/equipment/impe...ing_bryant.pdf

....and connect up at the top of your coax where it joins the longwire.

 

[kruser]

Good article and something I'll find handy but your link is broke.

Hmmm - It appears RR strips something for one reason or another as I tried to post the link I came up with and it also broke as can be seen below.
I'll add a screenshot of the link for those that are curious as to the link.

I googled the fragments of yours and came up with this http://www.dxing.info/equipment/impedance_matching_bryant.pdf which I assume is the same. Hmmm - also produced a broken link.

Thanks!

 

[ka3jjz]

Here are 2 sources of articles you might find useful...

Downloads | Antenna articles | Proceedings

Radio Equipment :: DXing.info

The late John Bryant's articles are toward the bottom on that DXing.info site. I just downloaded a couple of them, in Word format

best regards..Mike

 

[kruser]

Thanks for the links Mike.

Perfect timing as well as my loop experiment ended in failure a few minutes ago so I was going back to the drawing board and see what I did wrong. The links will give me a good place to start!

Steve

 

[lanbergld]

Kruser, If you take a 470 uH inductor & stick that into the antenna terminal, then attach your longwire to the inductor, your MW reception will come alive. You can use inductor values of 220 uH or 330 uH also. Wire length isn't the whole story.

I hope that's helpful.


Larry

 

[n5bxl]

Random (short) lengths of wire at AM broadcast frequencies have extremely low radiation resistance; but that's where the signal is developed. A 15 foot straight wire at 740 KHz has about 0.1 ohms in series with about 71 picofarads capacitance, a very high impedance to present to a 50-ohm radio antenna input. A series inductor, about 700 microhenries here will knock out the capacitance and allow the 0.1 ohm source to feed the 50 ohm port. But add a homemade broadband RF transformer with about a 20:1 turns ratio (wound on something like an FT-114-43 ferrite core); and you will bring that resistance up to somewhere near 50 ohms for a very useful signal with no added amplifier. Due to stray inductance you might have to series tune the transformer's secondary a bit to peak the signal. 'Worked for me on an Icom IC-718 used for good times and for hamming. The old cars had even shorter "probe" antennas; and they were tuned and matched to work quite well.