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Author Topic: Educate me about antenna choice.  (Read 3530 times)

Offline tybrad

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Educate me about antenna choice.
« on: March 29, 2014, 2048 UTC »
I am looking to improve upon my uncoated, 30ga bare wire antenna on my DX-160.  It wraps the roofline of my house on a horizontal plane and is apprx 150' long, and ~20' above ground.  I live in a densely populated suburb and this antenna seems noisy, but no more so than the telescoping antenna on my SWF-7600 and does not receive t-h-a-t much better than the 7600.  I am thinking of doing one of two options in the next couple of weeks.

1.  A 250' speaker wire conductor into the woods beyond my back yard, strung tree to tree at about 15' above the ground.  Straight or bent around, as in a rhombic?

2.  Set up an inverted vee with each leg about 125' from a standoff at my roof's apex (23' above ground), down to the ground.

Do you see anything correctible, or in need of refinement, in either option?
Other options?

Thanks in advance!
Maryland, US
Realistic DX-160
Sony SWF-7600
Vintage audio enthusiast
Physics instructor

rdla4

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Re: Educate me about antenna choice.
« Reply #1 on: March 29, 2014, 2215 UTC »
1. I would get what ever kind of antenna away from your home. Much of the noise is probably self induced from your home.
2. I prefer an antenna that goes for a length, and then comes back to the coax. (forming a large loop if possible). I have found that when the antenna is some form of loop, it is quieter.
3.If you use coaxial cable, use a balun. That seems to keep the noise from riding in on your coaxial sheild

I think just getting it away from your house will be the biggest help.
« Last Edit: March 30, 2014, 0006 UTC by rdla4 »

Offline tybrad

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Re: Educate me about antenna choice.
« Reply #2 on: March 29, 2014, 2301 UTC »
1. I would get what ever kind of antenna away from your home. Much of the noise is probably self induced from your home.
2. I prefer an antenna that goes for a length, and then comes back to the coax. (forming a large loop if possible). I have found that when the antenna is some form of loop, it is quieter.
3.If you use coaxial cable, use a balun. That seems to keep the noise from riding in on your coaxial sheild

I thing just getting it away from your house will be the biggest help.

Thank you, rdla.
I think that forming the loop inductively suppresses noise and makes the antenna more directional.  Correct?
Also, I do plan on a coax feedline and will use a 75-300 balun at the receiver for whatever I choose to do.

If I make a loop off of a coax feedline, what serves as the ground plane?  Half of the loop (like a folded FM dipole)?

Any other thoughts?  Anyone?

Thanks,
Ty
Maryland, US
Realistic DX-160
Sony SWF-7600
Vintage audio enthusiast
Physics instructor

rdla4

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Re: Educate me about antenna choice.
« Reply #3 on: March 30, 2014, 0011 UTC »
I usually have one end at the center conductor, and the other to the shield-----but it has to be long piece of wire to make that work. I my case I have a balun up in the tree, once end of the loop on one terminal of the balun, the other end on the other terminal of the balun. Coac runs in to the house, and connects directly to the SDR-IQ SDR coaxial connector.

I think the DX160 has screw terms? Its been years, cant remember, so that may be a good place to put the balun


Offline Beerus Maximus

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Re: Educate me about antenna choice.
« Reply #4 on: March 30, 2014, 1537 UTC »
A few comments..

For the ICF-7600, adding external wire might cause more problems than it solves. Portables are usually optimized for the whip and long random length wires can introduce overloading. Not saying it's impossible to improve the performance of this radio with an outside antenna, but it might be challenging.

For the DX-160, it appears that it has a 50 ohm unbalanced (??) antenna input (with screw terminals instead of a newer style connector). It might make sense trying to run coax from the antenna input to a random wire that has some kind of impedance matching transformer. I'm thinking something like this:

http://www.parelectronics.com/swl-end.php

A simple dipole might also do just fine for general purpose listening. If you still find yourself stuck it might be worth investigating how sensitive your DX-160 really is these days. That radio could be upwards of 40 years old and in need of some work.
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Offline tybrad

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Re: Educate me about antenna choice.
« Reply #5 on: March 30, 2014, 1828 UTC »
A few comments..

For the ICF-7600, adding external wire might cause more problems than it solves. Portables are usually optimized for the whip and long random length wires can introduce overloading. Not saying it's impossible to improve the performance of this radio with an outside antenna, but it might be challenging.

For the DX-160, it appears that it has a 50 ohm unbalanced (??) antenna input (with screw terminals instead of a newer style connector). It might make sense trying to run coax from the antenna input to a random wire that has some kind of impedance matching transformer. I'm thinking something like this:

http://www.parelectronics.com/swl-end.php

A simple dipole might also do just fine for general purpose listening. If you still find yourself stuck it might be worth investigating how sensitive your DX-160 really is these days. That radio could be upwards of 40 years old and in need of some work.

Thanks, beerus.
I am planning on only using this antenna on the 160 upon which I have done the sensitivity, RF grounding, and audio recap mods on this receiver.  I also put a digital freq counter on it.  This is what I did (from antiqueradios.com)

2. Use the fiberboard cover off of the rear of radio, as a pattern to fabricate a new cover from sheet metal. I used a sheet of plated steel from the bottom of a discarded video tape player. It had a few nice narrow vent slots punched in it, which I used. Use any sheet steel, copper, brass (roof flashing is great), or aluminum. Choose a thickness from .020 in. (0,5mm), to .040 in. (1,0mm). Venting is nice, but not really needed, as this radio does not produce much heat. Remove any paint from the inside of your metal cover, where the screw holes are. Grounding is the key to effectivness. I did not fabricate a metal shield, just used a quadruple layer of aluminum foil.

3. The new metal back cover will drop the sensitivity of the radio 2-4 'S' units unless the rod antenna bar is moved away from your new cover. Remove the 4 screws that hold the AM rod antenna to the metal chassis. Move the antenna about 1 inch (25mm) towards the radio front, and another 1/2 inch (13mm) to the closest side (left side viewed from back). Use a pencil, and use the screwholes in the plastic stands to mark the chassis. On mine, 2 holes went where the near-middle ridge in the chassis is. Look underneath, and be sure nothing is in the way. Drill new holes (3/32inch or 0,9mm dia.)DON'T GO THROUGH TOO FAR. Re-install the rod antenna in the new place. If you don't want to drill holes, rough up the metal chassis with sandpaper, and use a couple of dabs of a good industrial Epoxy adhesive. Once the rod antenna is secured, replace the top cover and install your new metal rear cover.

4. Now, to improve reception, remove R-2 and replace it with a 5K ohm resistor and a salvaged neon light bulb in parallel. These lights were used in wall switches, nite-lites, toolbox electrical outlet test lights. The original 220 ohm resistor acts as an antenna signal direct shunt to ground to help the protection diodes with static electricity. The neon bulb will short away any static over 60 volts, and the 5K resistor will handle anything below the diode threshold. The 220 ohm resistor killed off about 3 'S' units of signal on ALL bands. I like all the gain I can possibly get.

5. If you listen to the lower frequencies, and find that the antenna tuning capacitor is lacking in capacitance, short out capacitor C77. This capacitor is wired in series with the tuning capacitor, it does nothing but limit the maximum tuning capacitance of the input circuit for the first stage R.F. amp F.E.T.. Shorted out, the tuning will be more efficient, especially on the 'A' band. Some antennas will not need the extra capacitance, so you may not see any change except for the knob changing position for the same tuning effect.


I have not had the tuner aligned as it -seems- to be OK.  I think that an alignment cost would provide diminishing returns, but not sure.
« Last Edit: March 30, 2014, 1830 UTC by tybrad »
Maryland, US
Realistic DX-160
Sony SWF-7600
Vintage audio enthusiast
Physics instructor

Nella F.

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Re: Educate me about antenna choice.
« Reply #6 on: March 30, 2014, 2051 UTC »
A few comments..
For the ICF-7600, adding external wire might cause more problems than it solves. Portables are usually optimized for the whip and long random length wires can introduce overloading. Not saying it's impossible to improve the performance of this radio with an outside antenna, but it might be challenging.

Beerus is correct about portables. However, I also own a 7600 (bought back in the mid '90's) & there is an input jack for an external antenna. A 250' long wire antenna is every dx apartment dweller's wet dream (well @ least mine). Still, probably a bit long for anything other that a quality table top receiver. Never the less, half the fun in antenna construction & installation is in experimentation. I envy you. Good luck.
additional: I see (google image) mine's  a 7600"G"... maybe yours doesn't have an input jack.
« Last Edit: March 30, 2014, 2105 UTC by Nella F. »

Offline BoomboxDX

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Re: Educate me about antenna choice.
« Reply #7 on: March 31, 2014, 1014 UTC »
The only thing I would add is consider using a simple antenna tuner with your DX-160 to reduce images below the SW broadcast bands. I made one with a coil wrapped on a plastic spice bottle and a couple of old tuner caps and it did the job fairly well.

In the SW broadcast bands themselves I never encountered many images -- at least none I can remember.

Also there is a resistor across the MW RF tuning coil you may want to remove if you use your DX-160 to DX the AM band. It's R62, marked 33K ohm on the schematic. When I removed it from mine there was a definite improvement in MW performance.
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The usual Realistic culprits on SW (and a Panasonic).

Offline tybrad

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Re: Educate me about antenna choice.
« Reply #8 on: April 20, 2014, 1915 UTC »
Yeah, I removed it as part of the DX160 mods that are on the web.  Thanks!
Maryland, US
Realistic DX-160
Sony SWF-7600
Vintage audio enthusiast
Physics instructor

Offline ka1iic

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Re: Educate me about antenna choice.
« Reply #9 on: April 22, 2014, 1814 UTC »
"Also there is a resistor across the MW RF tuning coil you may want to remove if you use your DX-160 to DX the AM band. It's R62, marked 33K ohm on the schematic. When I removed it from mine there was a definite improvement in MW performance."

What was the reason for this resistor?  Perhaps to make the coil more 'broadband' or a sort of matching gimmick?

I use to use Faraday shields on the input coil of my old tube radios and it worked quite well but to place one in this radio might not be possible(?)  just an idea...

have a great day all :-)

73 Vince
KA1IIC
 

73 Vince
KA1IIC

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Offline BoomboxDX

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Re: Educate me about antenna choice.
« Reply #10 on: April 24, 2014, 1516 UTC »
My uncle, who was a ham and a ship's radioman, said he thought the resistor was to lessen the Q of the circuit, to 'broadband' it, perhaps for audio reasons.
An AM radio Boombox DXer.
+ GE SRIII, PR-D5 & TRF on MW.
The usual Realistic culprits on SW (and a Panasonic).