3/4 wavelength Short Wave receiving antenna?

[JasonW]

Hello All,

I've been looking into building a 3/4 wavelength wire antenna (with a 1/4 wavelength counterpoise) for receiving 41 meter band Short Wave broadcasting stations. (Like a 1/4 wavelength antenna, a 3/4 wavelength antenna is self-resonant and doesn't require a matching network as a high-impedance 1/2 wavelength antenna does.)

Where is the highest current point on a 3/4 wavelength antenna (in terms of wavelength) in relation to the feedpoint? This would be the best section of the antenna to mount as high as possible, since maximum radiation (or radiation reception) occurs here.

Many thanks in advance to anyone who can help. -- JasonW

 

[tpt]

About 20 feet off the ground should work well.

If you've got a big yard, great! You then can go higher to clear obstructions. But once you get this far off the ground and away from buildings, trees, etc., it should begin to work reasonably well.

 

[JasonW]

> About 20 feet off the ground should work well.
>
> If you've got a big yard, great! You then can go higher to
> clear obstructions. But once you get this far off the ground
> and away from buildings, trees, etc., it should begin to
> work reasonably well.

Oh yes, I can easily manage that height with an upward-sloping wire to a tree in the yard.

What I'm wondering is whether the maximum current point is closer to the feedpoint or the far end (I'm hoping it's the latter)? -- JasonW

 

[xoanon]

>
> What I'm wondering is whether the maximum current point is
> closer to the feedpoint or the far end (I'm hoping it's the
> latter)? -- JasonW
>

The current maximums are at the feedpoint and at the 1/2 wave point. A voltage max occurs at the far end (3/4 wave) - the current has noplace to go!

 

[kb5njd]

> >
> > What I'm wondering is whether the maximum current point is
>
> > closer to the feedpoint or the far end (I'm hoping it's
> the
> > latter)? -- JasonW
> >
>
> The current maximums are at the feedpoint and at the 1/2
> wave point. A voltage max occurs at the far end (3/4 wave) -
> the current has noplace to go!
>

I am crazy wrong on this one -- sorry guys. I just pulled out a model and it wasnt what I thought it was. Thats what I get for going from memory. My apologies.

john KB5NJD
<P ID="edit"><FONT class="small">Edited by kb5njd on 10/27/05 06:23 PM.</FONT></P>

 

[JasonW]

Thank you. It should work fine for receiving. Being so long, it will have a large signal capture aperture and the high current points should be far enough away from the feedpoint to get them up high. -- Jason

> > >
> > > What I'm wondering is whether the maximum current point
> is
> >
> > > closer to the feedpoint or the far end (I'm hoping it's
> > the
> > > latter)? -- JasonW
> > >
> >
> > The current maximums are at the feedpoint and at the 1/2
> > wave point. A voltage max occurs at the far end (3/4 wave)
> -
> > the current has noplace to go!
> >
>
> I am crazy wrong on this one -- sorry guys. I just pulled
> out a model and it wasnt what I thought it was. Thats what
> I get for going from memory. My apologies.
>
> john KB5NJD
>

 

[CW]

Uh Jason,
you are trying to overkill the technical here..On RCV, just about anything will do as long as it has enough capture area to get sufficient voltage for the rcvr to have a decent signal to noise ratio. As for "High Current" points, that would only be an issue if you were putting transmit POWER into the antenna, not using it for receive...on receive, the amount of signal present does not cause any current or voltage issues on a wire antenna (In other words, you can touch it! ....now, put watts of RF into it, and that becomes another issue...

If you put up a LARGE circumference horizontal loop as high as you can get it, that will work fine and have no directivity....A 3/4wl antenna for BC band is a bit LONG and thus in vertical mode would require a higher mounting point than found on most residential lots (say a couple of hundred feet)..


> Thank you. It should work fine for receiving. Being so
> long, it will have a large signal capture aperture and the
> high current points should be far enough away from the
> feedpoint to get them up high. -- Jason
>
> > > >
> > > > What I'm wondering is whether the maximum current
> point
> > is
> > >
> > > > closer to the feedpoint or the far end (I'm hoping
> it's
> > > the
> > > > latter)? -- JasonW
> > > >
> > >
> > > The current maximums are at the feedpoint and at the 1/2
>
> > > wave point. A voltage max occurs at the far end (3/4
> wave)
> > -
> > > the current has noplace to go!
> > >
> >
> > I am crazy wrong on this one -- sorry guys. I just pulled
>
> > out a model and it wasnt what I thought it was. Thats
> what
> > I get for going from memory. My apologies.
> >
> > john KB5NJD
> >
>

 

[JasonW]

> Uh Jason,
> you are trying to overkill the technical here..On RCV, just
> about anything will do as long as it has enough capture area
> to get sufficient voltage for the rcvr to have a decent
> signal to noise ratio. As for "High Current" points, that
> would only be an issue if you were putting transmit POWER
> into the antenna, not using it for receive...on receive, the
> amount of signal present does not cause any current or
> voltage issues on a wire antenna...

I'm trying to receive 41 meter band Short Wave stations in a house that has a metal roof, with nearby RF-noisy electric power lines. In this situation, knowing where the antenna's high-current points are and getting them in the clear as much as possible will help. I need to use an end-fed wire antenna because I don't have the space or supports (just one tree in a corner of the yard) for a dipole or loop. -- JasonW

 

[CW]

> I'm trying to receive 41 meter band Short Wave stations in a
> house that has a metal roof, with nearby RF-noisy electric
> power lines. In this situation, knowing where the antenna's
> high-current points are and getting them in the clear as
> much as possible will help. I need to use an end-fed wire
> antenna because I don't have the space or supports (just one
> tree in a corner of the yard) for a dipole or loop. --

Nope...you are wrong..if you have a noisy power line, you need to get that fixed by the power company....NO antenna will not fix this except maybe a shielded loop that is directional...and can null the power line noise..
The high current points again only matter for TRANSMIT....not receive and surely not in your case..I fail to see how it matters..a 1.4wave for 41 meters will work just fine...but the noise will still be there..Call the electric company and get them to fix the problem...otherwise, all you will have is high S meter readings because of the noise off the line..

BTW you can put up a V or somewhat inverted V with one support...fed with 75ohm coax, that would pick up fine on any SW band with a decent receiver...

 

[JasonW]

With all due respect, CW, you aren't familiar with the surroundings at my QTH.

I tried an end-fed 1/4 wavelength wire antenna. Pointing it away from the noisy power line (it's actually the transformer) did almost completely eliminate the noise, but that put the antenna in proximity to my metal roof, which resulted in much weaker signals.

Since antennas have reciprocity (the receive pattern being the same as the transmit pattern), the high-current points that radiate most of the signal when transmitting should also pick up most of the signals (the "sweet spots," if you will) when receiving.

With a 3/4 wavelength antenna, I can get these high-current points on the antenna pointing away from the noisy transformer (and further away from it than with the 1/4 wavelength antenna), plus get them (and most of the antenna) well away from the metal roof. I have to use an end-fed aerial for neighborhood political reasons (no CC&Rs, but I'd rather not get in fights with the neighbors over this, even though the law is on my side). An inverted Vee's new center support and coax feedline in the yard would draw attention that a #19 gauge black "invisible" stealth wire won't (they never saw my 1/4 wavelength antenna). -- JasonW