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NERVOUS
01-02-2010, 02:01 PM
Hello all.
In an attempt to lure me into the world of radio a friend bought me a shortwave radio for Christmas. I'm having a blast with it, but the reception could be better. The guy who gave me the radio suggested a long wire antenna. I got out my soldering gun and put the correct plug on the end of the wire and strung a couple hundred feet of wire from the trees using short chains of nylon zip ties to dangle the wires below the branches and keep them from touching anything.
The signal improvement is amazing. In fact it's a bit TOO amazing. It sounds like there's at least 4 people transmitting on every frequency and usually in at least two languages. I talked to my ham radio friend and he said the problem I was having is that the radio didn't have an antenna attenuator built in so I could filter out the stuff I didn't want.
Here's the question, I have a soldering gun, a shop full of tools, and some spare time, could I built an antenna attenuator? Does anyone have any suggestions of where to look for plans?
Thanks :D
Jeff

K8ERV
01-02-2010, 06:12 PM
A simple way may be to just put a variable cap in series. Would not be constant on all bands but might see if this works and is the problem

TOM K8ERV Montrose Colo

G3TXQ
01-02-2010, 07:36 PM
Take a 1kOhm or 500Ohm potentiometer. Connect the antenna to one end of the pot, ground the other end, connect the slider to the input of the receiver. Adjust the pot to the lowest level that lets you hear the wanted station clearly.

73,
Steve G3TXQ

NERVOUS
01-02-2010, 11:43 PM
Thanks K8ERV and G3TXQ for the suggestions. I'm having fun messing with this stuff :D


A simple way may be to just put a variable cap in series. Would not be constant on all bands but might see if this works and is the problem

TOM K8ERV Montrose Colo

By "cap" do you mean capacitor? Sorry if that question is a bit lame. So far my electronics experience is plugging circuit boards into slots and that's about it. My soldering experience comes in the form of my habit of making solder wire connections rather than using crimp connectors or wire nuts. It only takes an extra minute to solder it and it has always seemed to be a more reliable connection.

Thanks again,
Jeff

KX0Z
01-03-2010, 06:01 AM
Yes he means a variable capacitor.

K8ERV
01-03-2010, 02:28 PM
I can't spell capysittor. So I use "cap".

TOM K8ERV Montrose Colo

NERVOUS
01-03-2010, 04:17 PM
ok, got it. Thanks :D

W4PG
01-03-2010, 06:54 PM
The signal improvement is amazing. In fact it's a bit TOO amazing. It sounds like there's at least 4 people transmitting on every frequency and usually in at least two languages. I talked to my ham radio friend and he said the problem I was having is that the radio didn't have an antenna attenuator built in so I could filter out the stuff I didn't want.

Jeff, it sounds to me what you are really describing is a problem with selectivity and not sensitivity. A good receiver will have several filters for selectivity, where you can narrow the passband that signals come through. For instance, when listening on CW, which is a narrow signal, you can use 250 or 500 hertz filters to keep interference at bay. When listening to standard SSB, you typically use 3 khz, though you can narrow that down, but voice has a much wider range of frequencies and if you get much below 2 khz, the voice becomes very tinny and hard to understand.

Short wave broadcasts typically use AM transmissions, which for best listening, typically is set for 6 khz filters and higher. That same 6 khz filter, if used on CW, would allow potentially tens of stations to be heard at the same time; not good!

I have *never* run into a receiver that was TOO sensitive. You can easily adjust that by turning back the RF gain of the receiver (NOT that AF gain). Some receivers DO have attenuators built in, but typically that's only needed for local stations, like the ham next door.

It may be that the receiver you have, like many "short wave" receivers has no option for selectivity, in which case you are stuck with what you have and the receiver is likely meant to be used in AM mode only.

What brand of receiver are you using? That might help us to explain what is going on.

..............Bob

AD5ND
01-03-2010, 08:54 PM
Does the receiver in question have a knob labeled RF Gain. If so then try turning it down. If not try using a potentiometer like G3TXQ suggested. Its common for receivers to be overloaded by near by stations. Ecspecialy when connected to "a couple hundred feet of wire "

WB2WIK
01-04-2010, 12:00 AM
I can't spell capysittor. So I use "cap".

TOM K8ERV Montrose Colo

I have several caps, they all seem to have the logos of baseball teams on them for some reason...

NERVOUS
01-04-2010, 12:07 PM
What brand of receiver are you using? That might help us to explain what is going on.

..............Bob

It's a Grundig A6 http://milo.com/bag6-160-160
It's a small entry level radio. I'm having a blast fiddling with it, but it has very few features. I do understand that what I'm doing here is basicly hot rodding a chevy chevette. :D I'm enjoying fiddling with it anyhow.
I ordered the model 1054 kit from Ten Tec this weekend. I'll give a try at building that. If it goes well, I'll probably give that radio to one of the kids in the extended family and try building the Ten Tec 1254. http://store.tentec.com/kits/receivers
Check back with me in 5 years and maybe I'll be tackling an Elecraft :D
Thanks all,
Jeff

AI3V
01-04-2010, 03:41 PM
The attenuator may work, sorta kinda.

Trouble is it reduces ALL the signals, both the interfering and the desired.

What you really want to build is a preselector, basically, a preselector is a tuned circuit in the antenna lead that allows the desired signal thru with little or no attenuation, and has a relatively high attenuation off frequency.

A coil and variable capacitor,Tuned to the frequency you want to receive, connected in series in line with the antenna lead, or the same coil and capacitor wired in parallel between the antenna lead and ground should do the trick.

A ordinary "antenna tuner" is a preselector of sorts.

Rege

VK2TIL
01-05-2010, 12:48 AM
The problem sounds like overload.

This kind of receiver is designed to work well with its short whip antenna; it therefore has a lot of front-end gain.

Adding a larger antenna overloads the early stages of the receiver and increases undesirable products to the point where they form "phantom" stations.

A "supersonic heterodyne" (superhet) receiver operates by mixing and amplifying signals.

No amplifier or mixer is "perfect"; for example, theory says that, if we mix 15 MHz and 5MHz we will get 15 + 5 = 20MHz and 15 - 5 = 10MHz.

But here is the output of a mixer that has too high an input (RF) level;

http://img685.imageshack.us/img685/3032/sbl1mixera10dbm.jpg (http://img685.imageshack.us/i/sbl1mixera10dbm.jpg/)

(Frequency is across the bottom and output power is upwards).

We have our 10 & 20MHz signals but we have an awful lot of others; run this output through an amplifier or two and you will have every combination of signals you can think of!

Often two signals a long way from the desired one will mix to form another signal at the desired one; eg, if we just want 10MHz in the example above, 20 can mix with 30, 35 can mix with 45 etc etc.

Amplifiers are similarly-imperfect; even if the input is a "pure" sine wave (but even the "purest" wave we can generate has some harmonics) the output will have frequencies other than the "pure" frequency; this is again compounded if the input signal level is high.

The subject of reciprocal mixing is more complex than this necessarily-brief description.

An attenuator is a good aid when this problem arises; either the cap(acitor) or the pot(entiometer) will serve. The pot is probably easier for the OP to obtain.

Some radios of this kind have an attenuator in the form of a Local/DX switch but the A6 does not seem to have this.

A nice small radio but you gets wot you pays for; don't expect it to perform as well as a large communications receiver.

The Ten-Tec seems like a good project; it will perform very well if you take the time to learn to "drive" it.

AI3V
01-05-2010, 05:41 PM
If I can add a bit to what VK2TIL posted.

This intermodulation ("reciprocal mixing") he speaks of is quite common, and not just on inexpensive receivers.:(

For example, here at AI3V I use a Kenwood r-1000 for AM broadcast band reception. I had it hooked to a 30 ft "long wire" antenna, Reception was impossible on the 160M band (1.8-2.0Mhz) and below 550KHZ due to receiver overload, also, many stations in the AM band are unreadable.

My solution was to change the antenna to a small ferrite rod antenna. I wired a simple variable cap in parallel with the loop, tuning the antenna now provides enough selectivity in the preselector (in this case the antenna) That I can now copy 160M well, and a AM station every 10 KHZ (at night!:D).

Similarly, I have great problems on 6m(50-54mhz-Icom IC502),2m(144-148mhz-Icom IC251a),70cm(432-436mhz-Microwave Modules transverter) and 23cm(1296mhz-SSB electronics LT-23s transverter)

In each case I had to add a cavity type filter/preselector in order to be able to hear anything other than STRONG signals.

I live In the city, within a mile or so of 6 full power TV stations, and a boat load of AM/FM stations.

On some rigs the intermod appears as multiple signals on a frequency, on others the noise floor of the receiver went up dramatically. On another radio, the noise disappeared entirely, but so did all the signals!!

The classic test for receiver intermod is to tune in a signal, note the signal strength, now add a fixed known value attenuator in line,observe the signal strength again.

If the signal went down by exactly the same amount as the attenuator value you probably do not have intermod problems.

If the signal goes down by more than the amount of the attenuator then you have intermod problems.

Rege

K8JHR
01-17-2010, 12:09 AM
Mr. Nervous...

As a long time (40 years) SWL (Shortwave Listener) I am quite familiar with the problem you describe, and believe the correct answer is a shorter antenna. Another member said you have overloaded the rig, and I agree. A big antenna on such a small radio can easily cause overload the receiver. It is kinda like using a hearing aid at a ball game, it amplifies everything, so you hear everyone at once, not just selected individuals as you can when listening normally.

Just use about 40 feet of wire and it will be sufficient. SWLs have know this forever and it does not take a scientific treatise to diagnoze the problem or prescribe the cure.

Just MY take. Others' mileage may differ.

Happy trails. ========== James- K8JHR ==========

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