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Author Topic: yet another antenna question...near field effects?  (Read 3658 times)

Offline IQ_imbalance

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yet another antenna question...near field effects?
« on: August 22, 2015, 1535 UTC »
I know that the ideal is a high antenna away from anything else, but while I'm trying to decide on my next rig I'm just playing around with my 7600 and random long wires.  I have one strung along the back of a (wood) deck railing behind my house, and was thinking about running another at right angles to give me more options in terms of directionality.  The question is if I have the choice of running a wire under the tree canopy, or alternatively about halfway up and along the side of the house (without any trees overhead), which one is most likely to produce better reception?  Along those lines (pun not intended) is there any benefit to using a feedline since I'm just connecting the antenna to the radio via it's external whip?  I realize the answer might be 'try and see what works' but thought it would be worth asking.
LOG/NE-SW unterminated BOG
AFEDRI SDR-Net
Central MD

Offline BoomboxDX

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Re: yet another antenna question...near field effects?
« Reply #1 on: August 25, 2015, 0510 UTC »
I don't know that much about the Sony 7600's internal circuitry but if the radio has an external antenna input it may be better for the radio to use that instead of clipping the external antenna to the whip.

I know that with some radios the external antenna jack has protection diodes, where the whip antenna doesn't.

One zap of static electricity can wipe out an RF amp transistor.

The trees shouldn't affect your reception much, if at all. If you can get an antenna through the trees, and it will allow you do get a longer antenna out, then do that.

A wire will receive off the sides of the wire at SW frequencies even if it's as short as 50-60 ft (a half wavelength at 7 Mhz is roughly 60 feet, for example -- and a half wave antenna will receive off of both sides of the wire, with reduced reception off the ends of the wire). So keep that in mind.
An AM radio Boombox DXer.
+ GE SRIII, PR-D5 & TRF on MW.
The usual Realistic culprits on SW (and a Panasonic).

Offline Lex

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Re: yet another antenna question...near field effects?
« Reply #2 on: August 25, 2015, 1936 UTC »
If the 7600 has an external antenna jack like the 2010, try a simple loop instead of just more wire.  Best simple home brewed antenna I've found and works as well with my Sony portable as with my Palstar R30C.

It's just magnet wire stretched along a wooden fence into a vertically oriented square, 6-8 feet along each side - size isn't critical. I used an ordinary TV balun, although a proper 4:1 HF balun might work better.

Just strip the enamel from each end of the magnet wire and attach to the TV balun screw terminals.

I used ordinary TV cable with different adapters to fit the Sony portable and Palstar.

Simple, cheap, works great. Every random wire and low dipole I tried, either clipped to the whip or via the external antenna jack, picked up more local RFI and more QRN, especially summer static crashes. But the simple loop really improved the signal to noise.
That li'l ol' DXer from Texas
Unpleasant Frequencies Crew
Al: Palstar R30C & various antennae
Snoopy: Sony ICF-2010
Roger: Magnavox D2935
(Off-air recordings.)
Email=my name at hotmail dot com

Offline ChrisSmolinski

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Re: yet another antenna question...near field effects?
« Reply #3 on: August 25, 2015, 2008 UTC »
My experience has been the same, loops (and folded dipoles, which geometrically are somewhat similar to a loop) tend to pick up less local RFI/noise than dipoles or random wire antennas.

FWIW, I found a 7600-GR schematic here, I assume the 7600 models are pretty similar: http://stephan.win31.de/icf-sw7600gr.servicemanual.pdf
Chris Smolinski
Westminster, MD
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Offline IQ_imbalance

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Re: yet another antenna question...near field effects?
« Reply #4 on: August 26, 2015, 0334 UTC »
Hmmm the loop intrigues me.  Time to break out the thumb tacks and ladder.....
LOG/NE-SW unterminated BOG
AFEDRI SDR-Net
Central MD

Offline uhf35

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Re: yet another antenna question...near field effects?
« Reply #5 on: August 26, 2015, 1403 UTC »
My experience has been the same, loops (and folded dipoles, which geometrically are somewhat similar to a loop) tend to pick up less local RFI/noise than dipoles or random wire antennas.

FWIW, I found a 7600-GR schematic here, I assume the 7600 models are pretty similar: http://stephan.win31.de/icf-sw7600gr.servicemanual.pdf

Yes but, the 7600GR has an active antenna power supply band in it's antenna jack, that's not  protected by diode. If puts any standard plug, you are on risk of burn something.

Offline Lex

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Re: yet another antenna question...near field effects?
« Reply #6 on: August 26, 2015, 1701 UTC »
Yes but, the 7600GR has an active antenna power supply band in it's antenna jack, that's not  protected by diode. If puts any standard plug, you are on risk of burn something.

There's less risk of electrical damage with a short, passive antenna.  Those warnings usually applied to true longwire antennas - full wavelength antennas outdoors where a static charge could develop; and mismatched active antennas.  A small passive loop like the one I described should be safe.  I've used it on my Sony ICF-2010 and small Panasonic portables.  I disconnect it when not in use or if there's a risk of thunderstorms.

Also, an isolation transformer can be used, although the signal level will decrease.  There are plans available for making a simple connector with no direct electrical connection between the antenna and feedline to the receiver.  These are often used between true longwire antennas for 80m and longer to reduce the risk of static electricity damage. 
That li'l ol' DXer from Texas
Unpleasant Frequencies Crew
Al: Palstar R30C & various antennae
Snoopy: Sony ICF-2010
Roger: Magnavox D2935
(Off-air recordings.)
Email=my name at hotmail dot com

Offline IQ_imbalance

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Re: yet another antenna question...near field effects?
« Reply #7 on: August 27, 2015, 0141 UTC »
Any cons to using rg174/u coax as feed line?  It's much easier to snake through the window!!
LOG/NE-SW unterminated BOG
AFEDRI SDR-Net
Central MD

Offline Lex

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Re: yet another antenna question...near field effects?
« Reply #8 on: August 27, 2015, 0317 UTC »
Any cons to using rg174/u coax as feed line?  It's much easier to snake through the window!!

Should be fine, and the lighter weight cable is better suited to the smaller external antenna jacks on most portables.  I've used shielded audio cables as feed lines too, but only short term or temp use for outdoor antennas - I'd set 'em up over the weekend or overnight.

RG174 may pick up a bit more noise due to the lightweight shield, but sometimes this can be remedied with a simple choke: an air-coil choke of several coils of feedline wrapped around a form (I've used large empty vitamin bottles); or a snap-on ferrite choke.  Just experiment - sometimes a choke can make noise worse, it just depends on the local RFI-generators and the usual voodoo.
That li'l ol' DXer from Texas
Unpleasant Frequencies Crew
Al: Palstar R30C & various antennae
Snoopy: Sony ICF-2010
Roger: Magnavox D2935
(Off-air recordings.)
Email=my name at hotmail dot com

Online Pigmeat

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Re: yet another antenna question...near field effects?
« Reply #9 on: August 31, 2015, 0218 UTC »
Yup, I use a snap-on choke just below the feedpoint with rg-174. Two or three turns, snap it shut and tape it up.

If you've got an antenna tuner that accepts twin lead you can use lamp cord as a feedline. It's 50 ohms and the above choke trick works well with it. As it's flat it can go under the weather stripping on the window. Lamp cord/speaker cord/ tv line will react to nearby metal objects. If you've got a house with aluminum siding or a metal roof it might not be a good option for you.

I used to transmit with lamp cord or speaker wire antennas. One hundred feet of the stuff gives you two 33 feet arms for the dipole and 67 feet of twin feed line. Strangely, that just happens to match up with a certain band of frequencies.........

I don't know how many shows I used those dollar store speaker wire antennas for. When the feedline got too twisted, it got used for other stuff. Like snaring rabbits and rodents.