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Old 12th Feb 2024, 4:19 am   #1
Bazz4CQJ
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Default An 80 metre antenna for a *handheld* Rx

In trying to track down 80m RFI, I dug out my Alinco DX10 handheld scanner. It's the only portable Rx I have with signal strength indicator. I have the standard 150mm Alinco stub antenna originally supplied with it, but is there anything I could construct that would be better for listening on 80m? It has a BNC connector.

B
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Last edited by Radio Wrangler; 12th Feb 2024 at 11:25 am. Reason: 80m is actually HF
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Old 12th Feb 2024, 5:08 am   #2
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Default Re: An LF antenna for a *handheld* Rx

Couple of dozen turns on a ferrite rod antenna. It's also directional with two sharp nulls but unless you add something to make it asymmetric, the nulls are ambiguous.

DAvid
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Old 12th Feb 2024, 11:05 am   #3
AndyWright
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Default Re: An LF antenna for a *handheld* Rx

When I had this problem a few years ago, I used a small frame loop antenna (about 20" on a side) with a few turns of wire and resonated with a tuning cap. A single turn loop was used to couple to the rx. I then went to various locations in the car and took bearings by rotating the loop for best null then adding 90 degrees to get the peak.

These were plotted on an OS map. Three fixes were sufficient to locate a broken insulator on the 11kV lines in a field about a quarter of a mile from here.
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Old 12th Feb 2024, 11:31 am   #4
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Default Re: An 80 metre antenna for a *handheld* Rx

Thread title adjusted before you start getting any flak. There isn't any general name for groups of bands.

But nomenclature is a different and older problem, and this thread is about something to hunt RFI with (hint)

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Old 12th Feb 2024, 11:36 am   #5
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Default Re: An 80 metre antenna for a *handheld* Rx

A frame antenna is a good choice, and resonating it will help with overload issues with the lack of front end selectivity in hand held radios with broadband general coverage.

A second issue is in providing attenuation for when you're getting close. It gets difficult if the receiver screening isn't good.

David
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Old 12th Feb 2024, 1:40 pm   #6
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Default Re: An 80 metre antenna for a *handheld* Rx

When you get too close and the signal gets too strong so you can longer use the S-meter indication as a further guide to proximity, then as well as attenuation (as suggested by RW) you could also try listening to three times the affected frequency on the assumption that the source may be producing weaker, but still detectable, harmonics there.
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Old 12th Feb 2024, 2:24 pm   #7
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Default Re: An 80 metre antenna for a *handheld* Rx

Look at some of the direction-finding radios used for 80-Metre ARDF 'foxhunting' events;

https://openardf.org/ardf-open-equip...ceive-antenna/

https://ardf.fi/80m_en.html

Many use ferrite rods, and the general techniques used for locating a near-field transmitter will also apply to tracking down an indadvertently-transmitting noise-maker.

Just as directional as, and a lot easier to 'wield in the field', than large frames/loops!
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Old 12th Feb 2024, 2:33 pm   #8
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Default Re: An 80 metre antenna for a *handheld* Rx

Regular, ex-AM radio ferrite ought to be useful on 80m. On higher bands, losses will increase.

A carved up scrapper of a portable radio would yield a rod and a tuning capacitor to resonate it. Just remove long wave coil and pull turns off of the MW coil.

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Old 12th Feb 2024, 2:46 pm   #9
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Default Re: An 80 metre antenna for a *handheld* Rx

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Originally Posted by Radio Wrangler View Post
Regular, ex-AM radio ferrite ought to be useful on 80m. On higher bands, losses will increase.
David
"Normal " ferrite rods do indeed work OK on the lower HF-bands; the Roberts R707 uses a ferrite rod for its 41- and 49-Metre-band broadcast coverage [augmented by the telescopic whip].

There's a circuit for a [1.8MHz] DF receiver in the mid-70s version of the RSGB "Radio Communication Handbook" too - it uses a ferrite rod connected to a TAD100 IC - the antenna component values would no doubt scale to 3.5MHz without too much difficulty.
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Old 12th Feb 2024, 4:03 pm   #10
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Default Re: An 80 metre antenna for a *handheld* Rx

Many thanks for those ideas, both the rod and frame sound very useable. The noise here was high in the past but it is worse than ever now. I switched on the 718 the other day and it was S9 across 80m and no stations heard. The think the noise is also higher than it used to be on the magloop.

As someone who was never a listener on LW or MW, 160 and 80m were always "LF" to me, but the point is taken.

What time I've had recently has been devoted to going re-checking the G7FEK, re-soldering a couple of joints and replacing the SO239, and making some other changes to the support for the fibreglass pole at the centre and it will be back up maybe tomorrow, but late yesterday I found the pulley at one end has got stuck - may have to do some ladder climbing .

Some years ago, I did have an inverted V for 80, which I liked and seemed less-affect by noise than various horizontal wires which I had had. It was supported at the top at 60' up a Douglas Fir, but failed when the supporting rope broke. The tree- climbing had been done by a much younger friend, but I don't think I could ask him to do it gain. Also, the structure of Douglas Firs is not ideal for antenna support roles (close packed and near-horizontal branches). However, if the 7FEK does not resolve itself, I may try to seek out another volunteer to climb the fir .

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Old 13th Feb 2024, 8:43 pm   #11
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Default Re: An 80 metre antenna for a *handheld* Rx

I hate to ask this question, but connecting a long wave coil on a ferrite rod (no tuning cap) to the Alinco and that is very deaf; does it need a tuning cap? I assumed the noise was wide band, but the circuit still needs tuning?

I stumbled upon Paul's active ferrite rod aerial https://www.vintage-radio.net/forum/...read.php?t=579. That might be useful with the right LC combination.

I checked out the Alinco (not used for some time) using a 19" whip and that's fine, better than the ferrite rod on 3.5MHz.

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Old 14th Feb 2024, 3:46 pm   #12
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Default Re: An 80 metre antenna for a *handheld* Rx

A LW coil at 3.5MHz will look like a. RF choke.

Use maybe 25 turns of thin hookup wire wound in a single layer at the middle of the rod with a variable tuning capacitor across the ends of the coil.

Then wind on a coupling winding on top, maybe 3 or 4 turns, this connects to the A and E terminals of the radio.

If it is good to go you should get a noticeable peak in received noise at some point in the tuning capacitors rotation.
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Old 14th Feb 2024, 9:53 pm   #13
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Default Re: An 80 metre antenna for a *handheld* Rx

Also, remove the LW and MW coils from the rod you're using. It is likely that they will have spurious resonances around the 80m band and will give you odd deadspots.

David
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Old 15th Feb 2024, 8:18 pm   #14
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Default Re: An 80 metre antenna for a *handheld* Rx

Quote:
Originally Posted by G6Tanuki View Post
A LW coil at 3.5MHz will look like a. RF choke.
Use maybe 25 turns of thin hookup wire wound in a single layer at the middle of the rod with a variable tuning capacitor across the ends of the coil.
Then wind on a coupling winding on top, maybe 3 or 4 turns, this connects to the A and E terminals of the radio. If it is good to go you should get a noticeable peak in received noise at some point in the tuning capacitors rotation.
Thanks; I realise my mistake. The long wave coil measured (after I'd used it ) was some mH's inductance, resonating (theoretically) on 80m with just 0.4pF.

I've since found a longer rod (200mm) with no windings at all on it, and that shows ~0.7uH per turn (@10kHz). I've even found an old Dielecon (?) tuning cap which will make for a light and compact assembly.

Thanks

B
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Old 15th Feb 2024, 8:45 pm   #15
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Default Re: An 80 metre antenna for a *handheld* Rx

Remember that inductance goes up as turns squared.

David
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Old 15th Feb 2024, 9:24 pm   #16
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Default Re: An 80 metre antenna for a *handheld* Rx

Stamp-collecting must be sooo much less demanding .

B
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Old 16th Feb 2024, 1:08 am   #17
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Default Re: An 80 metre antenna for a *handheld* Rx

Built as per Tanuki and seems to work well. Peaked the trimmer (used a 10-80pF trimmer rather than the Dielcon), the winding was 60uH which needs ~33pF to get 3600.

On 3615, I can detect stations talking, but it seems that I can do that only when Alinco set to SSB, and I cannot resolve them (they are weak), and I think they disappear when I switch to AM.

Will do more tomorrow.

B
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Old 16th Feb 2024, 2:01 am   #18
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Default Re: An 80 metre antenna for a *handheld* Rx

Most speech activity on 80m is SSB. So that seems OK.

David
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Old 16th Feb 2024, 5:40 am   #19
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Default Re: An 80 metre antenna for a *handheld* Rx

The specs for the DX-10 omit any bandwidth figures, and I suspect that for a handheld scanner, going to 2000MHz, they could be a bit wide. So, the fact I can hear a SSB transmission on 3615 may not mean much?

More importantly, the 5 bar S meter is proving useful. I had completely forgotten I had the DX-10 until recently; my only portable with an S meter. I could carry the Icom 718 with a small SLA, but sounds less appealing.

B
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Old 16th Feb 2024, 4:09 pm   #20
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Default Re: An 80 metre antenna for a *handheld* Rx

Sounds like you are making progress.

Listening for SSB signals is just fine, most of the day there is no double sideband full-carrier AM to be heard on 80.

Don't worry about the signal being weak, it is actually easier to detect the null on rotation of the antenna with a weak signal than with a strong one.
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