Many thanks to SWLing Post contributor, Jack Dully, who writes:
I am just wondering what portable receivers are more susceptible to overloading with long dipoles, say 60-70 ft.
I regularly use such and I have never noticed anything unusual happening with a Sony ICF-7600GR, Grundig G3, or PL-880 to name a few. I just ordered a Tecsun 680. Perhaps many of the newer radios have better AGC thresholds or more robust front ends but I really don’t know for sure.
[Also] how exactly do you know if your receiver (portable OR not) is being overloaded by too big of an antenna (ie. dipole, inverted V and the like) and will it damage your receiver? Is there still a way of using a large antenna to capture more distant stations safely, especially with good quality portables?
Thank you for sharing this question, Jack, and my hope is that SWLing Post readers can chime in with details and advice in the comments section of this post.
These are deep topics, but I’ll try to answer a few of your questions…
First of all, you definitely can harm a portable radio by hooking it up to a large antenna. Many portables have no means of protecting themselves from ESD (Electrostatic Discharge). By hooking a portable up to a long wire antenna, you can expose it to ESD which will essentially deafen your radio until you’re able to repair it. Indeed, this reminds me of an article from our archives regarding a Tecsun PL-600 ESD repair. Some radios do have built-in ESD protection (like the PL-680), but I’m not entirely sure it would offer protection from a particularly strong ESD pulse.
Symptoms of overloading can vary. Sometimes overloading can sound like background splatter and even popping. Sometimes you’ll hear “images” of broadcasters across the bands; muffled audio of a blowtorch station. Another sign of overload is when your signal meter jumps at the same time your receiver goes deaf. It’s as if your radio is simply overwhelmed by strong signals and it can manifest itself in odd ways especially since the AGC usually falls apart.
Like you, I’ve found that my Sony ICF-7600GR seems to be able to handle large wire antennas with no discernible overload. Also, the Tecsun S-8800 (above) is well-equipped to handle larger external antennas and even sports a proper antenna port on the back. I know Sangean ATS-909X owners who only use their radio with an external wire antenna and have excellent results.
Some portable radios are very sensitive with the built-in whip antenna, but fall apart if attached to a long wire antenna.
In general, the cheaper the radio, the less likely it has a front end and filtering that can cope with overloading.
Please comment with your experience regarding overloading. Have you found some radio models better than others at coping with blowtorch stations, for example? What do you do to protect your receivers from electrostatic discharge when hooked up to large antennas? Please comment!
I have a long wire aerial feeding into a balum and an inline lightening discharger that is earthed but the coax run is quite long and the signal strength is poor. So yesterday I dug out my old Datong AD270 indoor antenna and housed it in a waterproof box with two old fiberglass car aerials to make an active dipole. I fixed it horizontally onto my mast with a stainless steel jubilee clip and It has the same length of coax run to the receiver but the signal to noise ratio is improved and signal strength is greater. So it was well worth a days work.
Hi looks like the main problem is new small portable radios. At home with portable l use Wellbrook loop,or the small set of AOL loops and ferrit rods for low bands and MW LW from home in England lve heared most Countrys at diffrent times of the year.
Playing a portable radio against aRA17 or RA1792 or my boat ancor AR 88 they are worlds appart l think every SWL should try a 1940s 1960s Valve recever they make the hobby fantastic.leave the portables for trips holidays. BOB M3DPQ ENGLAND.
If using 75-ohm coax and f-connectors, fixed value attenuators for CATV can be sourced affordably via eBay and similar. Get a few different values. There could be some drift in the actual attenuation values below the rated frequency ranges, but otherwise, most I have tried work fine at HF and usually even lower.
Want something a little nicer? Buy or build a basic variable attenuator. I sometimes use a RadioShack 15-678 “TV” variable attenuator between my 31′ vertical and shortwave portables. Its works nicely at MW, HF, and FM BCB with my PL-330, which lacks RF gain control or even a basic switchable fixed value attenuator.
I havent read all the replies and comments but one thing I would say concerning over loading at the antenna socket from a long wire outside antenna is if your receiver has a RF gain switch or varible gain control,turn it down so that any”chipping” or over loading sound is cut off,but still able to receive your SW station.The other important thing to remember is if your receiver has an antenna tuner or attenuator switch,adjust it accordingly to give good results.Many old hands at DXing and SW listening will know all the tricks to give best receiption results.One more thing,at Radio junk sales you can always pick up broad band antenna tuner,and this will work fine to stop any over loading. Hope this helps.
Depends. A transmatch – there are very few so-called ‘antenna tuners’ that work for SWL – is designed to transfer the maximum amount of energy from the antenna and feedline to the load (your receiver). While it does have a certain amount of attenuation when it isn’t tuned to the correct frequency, it might not be enough to solve your issue. A passive preselector is a far better choice in this case.
I use a Tecsun S-2000 (Satellit 750) and found it to be so sensitive it almost always overloads when connected to anything other than the built-in whip. The solution I found was to build a home-brew high-pass filter with a 2MHz roll-off. Almost all overloading at HF is caused by AM broadcast band images and intermodulation products. Even with the filter connected I still have to adjust the attenuator and/or RF gain to prevent it getting swamped. I have two setups: in the park I use a longwire > ATU > hi-pass > radio. At home I use amplified loop > hi-pass > radio.
The question of using a 60-70 ft dipole with a portable is interesting. If the radio only has a local/dx switch you may find it still overloads on the local setting: I found this to be the case on my Degen (Kaito) 1103. A better quality portable with adjustable attenuation (up to 30db) might work better. I sometimes connect a longwire by winding a few turns around the base of the whip so it’s inductively coupled, rather than a direct connection. Generally, less turns means less overloading, but the radio is still sensitive enough to pick up signals.
Adding some front end filtering using a passive (not active) preselector such as the one below will often help where the front end is getting wiped with AM (and sometimes FM and TV too) signals. SDRs such as the RTL-SDR (the ones that cover HF), Funcube Dongle and even the popular SDRPlay will also benefit.
Of course a combo that causes overloading in one location might not have it at all in another. It is very dependent on what you have around you in terms of AM, FM and TV sigs. Sometimes any long unshielded leads can act as unwanted antennas. It does sometimes take some detective work to get such a nasty issue under control
One thing that I thought of last night – if you can remove that whip when you are using the external antenna, do so. If you can’t just collapse it as far as you can. That switch that selects the external and internal antennas likely is just a cheap DPDT type with little or no isolation. This means that while the book says the whip is disconnected when using the external antenna, more likely it’s just switched out of the circuit. The whip and amp are likely still hot even when switched out, and with enough RF in the area, coupling at the switch is not out of the question. That might be one reason why your filter isn’t as effective as it could be.
I usually use two external antenna types for shortwave reception at home. I live in a suburb of a small city in Northern California on a lot that measures 60 x 100 ft (18 x 31m). There are not many local transmitters nearby that can contribute to overloading on the shortwave bands. There is one 50 KW FM transmitter and tower located 1.9 miles away. Both external shortwave antennas that I have used during recent years are passive wire antennas 12 to 25’ off the ground. The random wire/long wire was 105’ in length and the other is a full-wave horizontal loop at 83m. The long wire was in use for years but several years ago I simplified my antenna choices and took it down in favor of the less noisy horizontal loop. The horizontal loop is not a small loop and uses 273’ of wire for the loop element.
Regardless of the radio in use both of these antennas offer shortwave performance far better than can be achieved by using built-in telescopic whip antennas indoors or outdoors. 50 or 75 ohm coaxial cable is used as a lead-in line with these antennas. They connect to the external antenna jacks on my portables with BNC to 3.5mm mono plug adapters.
The external antennas are unplugged from the radios when not in use. This reduces static charging from wind and is a good but far from foolproof safety measure should an unexpected electrical storm appear. Removing plugs from an antenna jack when not needed keeps the springs contacts in the jack from taking on a set: which can be a problem with inexpensive radios. Prior to plugging an antenna into a radio any static charge is dumped by touching the tip of the antenna plug to the bare metal chassis of my grounded Hammarlund SP-600 radio or to the tip of my tongue.
Not all portables are equal when it comes to performance with external antennas.
I have a Radio Shack DX-399. This is a little jewel of a radio from the late 90’s. It is a rebranded Sangean ATS-606. It’s a jewel when used with the whip antenna or a Sangean ANT-60 23’ reel-up antenna but it overloads terribly with a random wire much over 30’ in length. This radio has no S-meter but there is a red LED indicator that lights when a strong station is tuned. With a 100’ long wire this radio is totally overloaded. The red LED lights constantly across the entire shortwave spectrum whether a station has been tuned in or not. Random and distorted stations are randomly encountered on the dial. Moving the DX/LOCAL switch to LOCAL does not help.
I have three multiband portables that are unusually good when used with my external antennas.
Eton Grundig Edition Satellit. This is an earlier model of the current Eton Elite Executive. The two radios are identical but have slightly different cosmetic details. These Etons are difficult to overload with external antennas but they will overload on rare occasions with very strong signals and when band conditions are particularly good. There is a DX/LOCAL switch for overload relief but this seldom provides a satisfactory solution. If overloading when DX is selected the LOCAL setting will offer either too little or too much signal attenuation. Overall, due to features including small size and light weight these radios are a very good choice for use with external antennas, especially on those DX trips outdoors and if you have 300’ of long wire antenna on a spool.
My next two radios are the Sangean ATS-909X and the ATS-909X2. The later model adds some significant features and upgrades to the radio but none of these changes effect the excellent performance of these radios with external antennas.
These Sangeans are very difficult to seriously overload with very strong shortwave stations when external antennas are in use and RF GAIN is adjusted to MAX. The RF GAIN control on these Sangean radios is not a DX/LOCAL switch but a rotary potentiometer mounted on the left side of the radio. The control thumb wheel is marked MIN, 2, 4, 6, 8, and MAX. MAX is the default position for RF GAIN, which is turned down only to reduce overloading or to improve audio quality on very strong signals. I have experienced overloading during periods of excellent propagation with powerhouse signals from Radio Havana Cuba and China Radio International transmitted from next door in Cuba and from WBCQ when it was testing 500 KW in Maine. Other than that, these radios have been overload-free with RF GAIN set to MAX. These instances of overload were easily cleared by turning the RF GAIN control down to MIN and turning it up again until S-meter signal peaks did not exceed the top of the meter scale.
The less expensive the multiband radio is the more likely it is to be subject to overloading. An external antenna jack is one indicator of good performance with external antennas but most portables will overload with significantly more than 23’ of random wire, antenna jack or not. Other factors that influence overloading are the presence of local transmitters in the listening location and the performance of the external antenna.
The Sony ICF 2001 and ICF2021 are notorious for having that FET front-end ESD sensitive. Easiest way to get around that are to bridge a couple of 1N4148 diodes, opposite of each other polarity-wise, across the antenna center input and chassis ground. Many folks do this modification internally in the radio, but an external connector can easily be made to do so as well. That way, you can apply it to other portables. I did this for my Sangean ATS-803A portable. What will happen? If a big spike, or static charge, goes at the .7V level, it will not go any higher that that voltage. That, and if you should be super close to a transmitter, like a CB’ers driving past your QTH keying-up, that RF will not go higher than .7V. Now, a .7V RF signal will most likely cause front-end overload and IMD on most portable, but it should be a level that should not ZAP your front-end. Also, a close lightning strike will not spike higher than .7V. Anyway, that is a quick way, and cheap way, to protect your portables from ESD.
Nice try, but you should completely read the thread I linked in the original message.
The diodes also have another down side – if you connect a big antenna to a portable in an area with a very high signal level – like the NYC suburbs, for example – they could start to rectify and actually cause more noise. In a rural location, you probably wouldn’t have this issue. When I wrote that article with the little box up for RadioReference, I had 1 or 2 folks tell me that’s what actually happened. Substituting a small gas discharge tube (I think that’s what they used) solved the issue
I’ve heard grounding can actually make wonders against ESD. In fact It’s a must when pairing devices with external antennas: both for protecting and signal clarity.
Not always true. In fact, just ramming a ground rod into the ground – which I’ve seen recommended on far too many user manuals – might be the very worst thing you can do. There have been innumerable discussions on this very topic, so trying to summarize them in a short reply is next to impossible. Suffice it to say that in a thunderstorm, your best bet to protect a radio is to disconnect the antenna from the radio (and ground it away from the home as much as possible).
With portables without an antenna jack, this won’t work very well in any case.
In fact, I’ve read many accounts of using a ground rod making matters worse – not better. If you live in the US, the NEC codes should always be your guide.
Before we can talk about ESD protection, let’s clarify one thing; there are 2 very common ways to connect an antenna to a portable; clip it to the whip (a VERY bad idea) or use the antenna jack (if it has one). The first way should be avoided at all costs; many times there’s a RF Amp near the base of the whip, and one little zap from static electricity can and quite possibly will blow the amp, rendering the portable deaf.
At least some portables have a couple of diodes at the antenna jack which would blow if the voltage gets too high, but as stated previously, that’s far from foolproof. Add to the fact that you are throwing a lot of RF from a big antenna into a radio that really isn’t designed for it, and overloading can be an issue. Some radios just have a DX/Local switch for an attenuator, and it sometimes ends up being too little or too much. An external circuit with a crude but simple RF gain control in the form of a pot (you would have to experiment to see what values work best for your environs) and a simple circuit as described here. Better to blow a little box you built than the amp in your radio. This topic is discussed here…