Confused About Impedance Matching

[radioetc]

Recently I have reconfigured a coaxial end fed antenna in an attic to a dipole fed with ladder line that goes directly to the balanced input of a vintage shortwave radio.

From what I read, the impedance of the center of the dipole is approximately 73 ohms but the ladder line is 450 ohms. Yet, there is no impedance matching required between the ladder line and the antenna. Why is that?

Also, it seems that it is recommended when feeding a dipole with coax, to use a 4:1 balun but I have also seen 1:1 recommended. But this does not seem to properly match typical 50 ohm coax (though TV coax would be closer in impedance).

Anyway, just confused on what the rules of thumb are for ladder line vs. coaxial fed dipoles going to a radio with a balanced input.

Lastly, doing a little experimenting I noticed that some frequencies come in better shorting one of the balanced posts to ground. On other frequencies, this makes reception worse. Yet on other frequenices, it makes little to no difference. Why is that?

 

[prcguy]

The impedance of a half wave dipole at its fundamental resonant frequency in free space is in the 70-75 ohm range. The same dipole in your back yard 20ft off the ground can be closer to 50 ohms. In your attic probably similar. Get just a little outside its very narrow resonant range and the impedance goes all over the place and where the individual elements are operating at a half wavelength or multiples, the impedance can be several thousand ohms. A dipole is very predictable when built for one specific frequency but for general short wave use its all over the place.

If you try and use the dipole where each element is a half wavelength or multiple and the feedpoint impedance is several thousand ohms, when you connect 50 ohm coax, instead of transferring the energy from the antenna to the radio efficiently it can load the antenna down and you can loose much of the signal depending on the length of the coax in wavelengths where it can become an impedance transformer, and when coax is used in a severe mismatch condition its rated loss goes through the roof. That adds up to an otherwise ok working antenna loosing much of the signal it picks up in loss getting to the radio.

Using a higher impedance and ultra low loss transmission line like TV twinlead or 450 ohm ladder line can help bridge the gap in wild impedances at the feedpoint and the super low loss will not loose as much signal under extreme mismatch conditions. Remember, your dipole is only a half wave dipole at one specific frequency and at all others is just a random length with varying impedance and other complex parameters.

Not knowing the schematic of the input circuit of your receiver its hard to say why some signals work better with one post grounded or not. When you ground one side of the ladder line at the radio you unbalance the entire antenna system and the whole thing including the feedline can become the hot antenna where ungrounded the feedline is just the feedline and should not radiate or contribute to reception. Maybe you are listening to lower frequencies that benefit from a larger antenna and unbalancing the antenna adds more wire and works better there? I dunno.

Recently I have reconfigured a coaxial end fed antenna in an attic to a dipole fed with ladder line that goes directly to the balanced input of a vintage shortwave radio.

From what I read, the impedance of the center of the dipole is approximately 73 ohms but the ladder line is 450 ohms. Yet, there is no impedance matching required between the ladder line and the antenna. Why is that?

Also, it seems that it is recommended when feeding a dipole with coax, to use a 4:1 balun but I have also seen 1:1 recommended. But this does not seem to properly match typical 50 ohm coax (though TV coax would be closer in impedance).

Anyway, just confused on what the rules of thumb are for ladder line vs. coaxial fed dipoles going to a radio with a balanced input.

Lastly, doing a little experimenting I noticed that some frequencies come in better shorting one of the balanced posts to ground. On other frequencies, this makes reception worse. Yet on other frequenices, it makes little to no difference. Why is that?

 

[radioetc]

Not knowing the schematic of the input circuit of your receiver its hard to say why some signals work better with one post grounded or not.

It sort of seems that shorting helps 160 and 80 meters, is of not much difference around 40 meters and hurts the signal going above that (I think). Shorting it also raises the noise level but it seems I get a little more signal than noise so it's an over all it's a minor improvement. So I keep an alligator clip on the ground post and short or unshort one of the antenna posts as needed. FYI...the antenna is resonate at 10.8 Mhz dividing the 43 foot length into 468.

What I really want to accomplish with this is being able to hear weak signals in the 160, 80, and 40 meter ham bands. Often there is a QSO and I can hear one or two people but somebody else is in the mud. And I realize conditions will govern this situation regardless of antenna but nonetheless I want to capture as much signal as I can. Shortwave broadcast is excellent in the 5-6 Mhz range but is weaker elsewhere and boosting the signal up higher in the broadcast bands would be nice. The way the radio and antenna is now, it is totally listenable...it just seems it could be a bit better so just trying to figure out if the reception symptoms I am observing suggest any tweaks one could make.

With regards to a schematic, here's the manaulslib link for the Hallicrafters SX-11 if you are curious. Download Hallicrafters SX-11 Super SKYRAIDER Operating Instructions Manual

Thanks for the response!

 

[prcguy]

For the lower bands like 80 and 160 you might try disconnecting the antenna, shorting the balanced line together at one post then ground the other post and add some long ground radials. Then the entire antenna will operate more as a vertical element with capacity hat over ground.

At a remote controlled site I have a low ZS6BKW dipole at about 25ft horizontal and a 43ft vertical on the same property. If I tune across 160m in the evenings with the dipole I might pick up two conversations. When I switch to the vertical I can hear a conversation about every 10KHz on the vertical and it hears tons of stuff the dipole misses. You would probably have to get a dipole close to a half wavelength high to hear very low angle stuff on 160m and that would be about 250ft and not practical. A vertical is much easier and can work fine for some stuff.

It sort of seems that shorting helps 160 and 80 meters, is of not much difference around 40 meters and hurts the signal going above that (I think). Shorting it also raises the noise level but it seems I get a little more signal than noise so it's an over all it's a minor improvement. So I keep an alligator clip on the ground post and short or unshort one of the antenna posts as needed. FYI...the antenna is resonate at 10.8 Mhz dividing the 43 foot length into 468.

What I really want to accomplish with this is being able to hear weak signals in the 160, 80, and 40 meter ham bands. Often there is a QSO and I can hear one or two people but somebody else is in the mud. And I realize conditions will govern this situation regardless of antenna but nonetheless I want to capture as much signal as I can. Shortwave broadcast is excellent in the 5-6 Mhz range but is weaker elsewhere and boosting the signal up higher in the broadcast bands would be nice. The way the radio and antenna is now, it is totally listenable...it just seems it could be a bit better so just trying to figure out if the reception symptoms I am observing suggest any tweaks one could make.

With regards to a schematic, here's the manaulslib link for the Hallicrafters SX-11 if you are curious. Download Hallicrafters SX-11 Super SKYRAIDER Operating Instructions Manual

Thanks for the response!

 

[radioetc]

and add some long ground radials.

Suppose one was to experiment with this idea but was reluctant to commit to the work involved unless the improvement could be guaranteed. And then suppose for a radial, one stretched out a 100 foot extenstion cord out the window and connected it to the radio as the radial in the fashion you specified. Would that work or would I need real radials buried in the ground or something?

Also, do you have any opinion on ferrite rod loopstick type antennas? From what I've read, if using using the right ferrite material and windings and amplfication, they an work up to 10 Mhz and maybe up to as much as 20 Mhz. Might a ferrite loopstick designed for 160 or 80 meters work better than a vertical? Or at least no worse than a vertical of the type you use remotely?

 

[prcguy]

One ground radial on a vertical makes an inverted V dipole sitting on its side with one radial laying on the ground. Better than no radials but worse than a bunch of them. At least try tying the ladder line together on one post and grounding the other.

I don't think a loopstick antenna would do much good except for nulling out noise in a particular direction. Without an amplifier they don't work very well. An amplified loop would work much better. Even with a good amplified loop they rarely work better that a lot of wire outside and at that point the only advantage they might have is the ability to null out unwanted signals.

Suppose one was to experiment with this idea but was reluctant to commit to the work involved unless the improvement could be guaranteed. And then suppose for a radial, one stretched out a 100 foot extenstion cord out the window and connected it to the radio as the radial in the fashion you specified. Would that work or would I need real radials buried in the ground or something?

Also, do you have any opinion on ferrite rod loopstick type antennas? From what I've read, if using using the right ferrite material and windings and amplfication, they an work up to 10 Mhz and maybe up to as much as 20 Mhz. Might a ferrite loopstick designed for 160 or 80 meters work better than a vertical? Or at least no worse than a vertical of the type you use remotely?