Pushing a SHORT Antenna With Loading Coil

[NQC]

Hey All,

Kind of between a rock and a hard place.

The only spot on my apartment building property to try a wire antenna is a run limited to 35 ft long and  maybe 9 ft up at  the far end sloping to around 6 ft up at the radio end.

I would like a temporary “ set up, use and then take  down “ type of arrangement for a GOOD double tuned crystal set Dx  use at night, every now and then.


These limitations present a pretty dismal prospect, especially for the low end of the broadcast band.But pretty much all I have to work with.

Ideally 100 ft long and 30+ ft up is what one wants for crystal sets.



“Generally” ,short antennas need inductance (and overly long ones need capacitance).

So I am not sure if there is any role for series variable capacitance anywhere in the feed line .

 

This antenna MAY just be crying out  for inductance only.

 But it as also WAY  off from optimal design specs .

So I am definitely open to other unforeseen factors,ideas and  viewpoints.

FWIW , the radio’s antenna tank coil can be switched for parallel or series variable cap .There is also a full range of coil Z taps for the feed line.

But in the case of a short antenna I understand parallel tuning the radio’s ant coil is the way to go.





Just looking for opinions and ideas as to whether a simple loading coil ( it must be located at the radio end)  is really going to perk things up enough to be worth my while.

I am thinking that trying to goose such a short and low antenna into “ some” measure of improvement simply by inserting a “magical “ / “cure all “ load may really not cut it.

IF ( IF) I did go for it, I would envision something BIG  diameter (2-3 ft ?) with the turns spread out as much as is practical , yet still have a decent inductance.A compromise of turn spacing would be necessary ( ie 1-3” ?).

Is my load design off base?

Maybe a half baked LONG shot, but does anyone have any ideas of even a prayer of improvement on such a crumb ball antenna?

I know receive antennas are FAR less critical vs. transmit antennas.

Yet I still kind of feel like I am really pushing the envelope.

Obviously, I can experiment on my own, but just looking for ideas from other folks.

My ground may end up being a 25 ft bare copper run laid in the garden and kept wet.

So at least the ground return may
be “ good” with a ( hopefully) low resistance.

ANY input is welcome.

de NQC

[Brian]

You could also try capacitance in parallel with the antenna.
Or how about a "slinky" antenna?

[NQC]

Hey Brian.

Thank you for the reply.

Re adding capacitance in “ parallel “ with the antenna: Since this antenna is only a single wire I can certainly see how I could wire a variable cap in series to it.

I am not sure how one would wire a var cap in parallel to a single wire antenna.

A var cap could, I suppose, be wired in parallel to the single winding loading
coil.Perhaps such an arrangement could / would influence the antenna itself.

But the antenna tank coil in the radio has a great deal of flexibility: it can have the var cap in series or parallel. It also has a wide range of taps for Z experimentation.

If, in a hypothetical, the load had a parallel wired var cap on it, it would seem to redundant to the functions that could be achieved in the radio’s antenna tank coil.

Since this is an occasional use endeavor it is preferable to keep the var cap count down to two- if all possible.

Re Slinky: I would like the actual 35 ft antenna run to be as thin and discrete as possible.The far end would be attached to the guy wire of a small telephone only wooden pole  ( no power lines on pole) .It is NOT authorized and technically, NOT supposed to be there.

A Slinky would probably give a bit more of a visual profile than I would like.Also , one is much better off RF wise with the brass ( or even gold) plated Slinkys.Tough to find and a little deeper than I want to go.

Anyways, it’s really fun to bounce ideas around .

OT , my people are from Galway.

Again, I REALLY appreciate your looking at my post.

Thanks,

NQC

[RobRich]

Admittedly I have about zero experience with crystal receivers. I suspect your receiver probably has a rather wide input impedance range, right? Still, have you tried an moderate-ratio unun transformer between the wire and receiver?

Electrically short HF antennas often have like a 4:1, 5:1, etc. unun at the feedpoint. The Comet 250b and similar are good examples. Analysis here:

https://web.archive.org/web/20230203010839/https://g8jnj.webs.com/cometcha250b.htm

Longwave enthusiasts using short passive vertical often higher unun ratios like 16:1 up to even 100:1.

Your 35' wire antenna is an ~1/16-wave antenna at 1700KHz and an ~1/56-wave antenna at 500KHz. I suspect a typical 4:1 or 9:1 unun might very well benefit your overall MW BCB reception, though perhaps more like a 16:1 might be even better for lower MW BCB down into LW. YMMV.


I have used a ~2.5:1 dual-core unun built similar to the Comet ~5:1 unun design with decent results for my 31' vertical, but admittedly that was typically in conjunction with a quite sensitive Airspy HF+D SDR. I have since migrated back to a 4:1 unun, which seems slightly better for my particular purposes. That antenna likely would have a 9:1 or even higher-ratio unun if I was really and truly concentrating on very low frequencies, but I tend to prefer HF listening. Also the current 4:1 unun suffices for reception of WWVB, a few LW beacons, and even the occasional LW broadcast station anyway.


Alternatively a low-gain, high-impedance preamp could be used to deal with the huge mismatch between your receiver and wire. You probably do not need much, if any appreciable gain given typical signal and noise levels at MW.

[NQC]

Hey RobRich,

Thank you SO much for your reply.

So you feel the best approach would be to try an Z match the antenna , such as  it is , rather than try and slap a “guesswork “ inductor in series to the feedline - yes ?

So maybe I will look into picking up some # 61 mix doughnuts and stack a few up.

A quick construction question: are the cold end of the primary AND the secondary bonded to the same grounding point? Could there be any advantage to using separate grounds ?


FWIW, I figure the UNloaded Z of the radio’s tank COULD be , as a GUESS,anywhere from 100K Z to maybe (?) as high as 200K Z ?

Would these figures help on the turns ratio estimations ?

I may also be able to increase the length of my buried ground wire to try and do better there. We’ll see if that helps.

There may be a local noise issue as well, so I will probably stay away from RF amplifiers. Even IF they can act as an efficient Z matching device, in addition to their amplification action .

Also , to be a “true” crystal set, RF amps are a no-no.

OT , please see my post in the Longwave section .

FWIW, my private messages do not work.

Thanks Again,

NQC

[RobRich]

Okay I suspected somewhat high impedance, but an extremely high 100k+ Z input impedance range likely negates any need for further impedance transformation unless you are using perhaps a low-impedance coxial feedline between the antenna and receiver, as that might benefit from a high-ratio step down at the antenna and a high-ratio step up at the receiver.

As noted I have about zero experience with crystal receivers over than knowing the basic circuit idea. Oddly enough I never even built a homebrew single-diode radio.

BTW, about baluns and ununs in general, there are several good DIY guides here:

https://vk6ysf.com/projects.htm

[NQC]

Hey RobRich,

Thank you SO  for all the info.

I REALLY enjoy bouncing ideas back and forth with folks.

de NQC

[Brian]

Hey Brian.

OT , my people are from Galway.

Again, I REALLY appreciate your looking at my post.

Thanks,

NQC

I'm not too far from Galway, about a 1.5 hour drive. I bought my previous car from someone there.