Radiation From Dummy Load - Information Share

[mbrun]

I thought I would post this little nugget of information for any technical nerds in the crowd.

 

Recently I was doing some experiments that necessitated creating a very low-level FM signal. Using what I had on hand I decided to try and see if I could achieve said signal by using a radio connected to a dummy load. Knowing that a dummy load is a poor antenna and it eats most incoming RF for dinner, I theorized I may be able to achieve my goal by using a radio set to low power and transmitting into it. So I tried.

 

In my testing, I set an HT set to 1/2 watt, connected it to a 100w dummy load and keyed up indoors. The resulting RF signal was too strong for my purpose. So I did this multiple times at various distances. It was not the low level result I was hoping for. So I moved outdoors.

 

When outdoors I did the same thing. Only this time I used a rubber band to key up the radio. I announced myself, put the radio in front of a NOAA weather radio in a box on the ground, then started walking and listening on my HT. To my surprise I received the signal reliably and with very good intelligibility out to 1/8 mile in my tree-dense area. I continued walking until I could no longer pick up signal at all. That was at 1/3 mile.

 

Curious, I called a local GMRSr and ham on the street to see if he could receive the signal. Indeed he could. Using his base radio and external antenna he was receiving the signal reliably 1/3 mile away but in the opposite direction.

 

So, if you have never done this, this should give you a sense of how much RF still leakage from your dummy load still gets out when you are are using it.

 

 

Michael

WRHS965

KE8PLM

[SkylinesSuck]

Wow. Very interesting! What freq was it?

[kidphc]

Not completely surprised. I mean I have read about guys with beer can, light bulb antennas and such Dx'ing.

 

Although, quite a bit further then I thought it would go with a dummy load. But it does have a perfect SWR. I know, I know J/K.

 

What frequency were you using?

 

Sent from my SM-G975U using Tapatalk

[mbrun]

[mention]SkylinesSuck [/mention][mention]kidphc [/mention] I was using 467.6125 MHz.

 

 

Michael

WRHS965

KE8PLM

[axorlov]

It's not just the dummy load that radiates, it's the whole system ht + cable + dummy load. You would need to have a good RF choke on your cable to judge the radiation just from the dummy load.

[mbrun]

It's not just the dummy load that radiates, it's the whole system ht + cable + dummy load. You would need to have a good RF choke on your cable to judge the radiation just from the dummy load.

Good point. Thanks. I could also use a slightly longer length of coax and coil it. I think also I could use quad shielded cable.

 

 

Michael

WRHS965

KE8PLM

[axorlov]

Longer cable will make the electromagnetic field from the cable stronger. The coiling may or may not make it stronger, but will not reduce it on UHF. You need to suppress the common mode current on the outside of the cable shield (this is why quad shield will be no difference). On UHF this suppression is not as trivial as on VHF or HF. The 1/4 wave stubs are often used, but they are very narrow filters. Another approach is to have several ferrite beads (4-7)  on the outside of your cable.

[mbrun]

Understood. Since I like to experiment I sense a number of different scenarios in my future.

 

Thanks again for input.

 

 

Michael

WRHS965

KE8PLM

[OldRadioGuy]

I agree with people who suggested it is probably common mode radiation from the coax or the whole radio.

Put some big UHF ferrites on the coax & power cable and you'll probably see it go way down.

 

Vince

[Citizen]

Put some big UHF ferrites on the coax & power cable and you'll probably see it go way down.

 

How does one know which type/size/number of Ferrite Beads to use, and where along the coax and power cable to place them?

 

For example, I have 36' of LMR400 from my UHF/GMRS radio to the antenna.  The power cord is whatever came with the mobile radio (standard cigar-lighter plug type), I think maybe 14g wire, probably 3' in length.

 

Thanks.

 

...

[OldRadioGuy]

There is no such thing as too much ferrite so get the biggest ones you can tolerate.

You can use on or more on each cable.

Usually placement along the cable is closer to the source but it's not critical.

 

Ferrites are charactertized but frequency range.

So you want want intended for 460MHz.

If you go on digikey there should be a filter to select ferrites by frequency range.

It's not an exact thing.  Just get close.

 

You've seen ferrites on some USB cables and they are like the size of a Pecan or a bit smaller.

So this is a good size to look for.

 

You probably want "split" ferrites which can be opened up and placed on the cable without removing connectors.

They have a hinged plastic shell that snaps together.

They can easily be moved or swapped around to try different locations.

 

On the power line you can pass the leads through twice if there's room.

Just loop right around and come through again the same direction.

 

Any ferrite that is intended for UHF will probably work.

 

Here is an example of one to check out.

https://www.digikey.com/en/products/detail/fair-rite-products-corp/0446164281/8599533

 

The data sheet will give the exact attenuation curve.

There is an oval button near the top to download the data sheet.

Here is the data sheet.

 

file:///C:/Users/19712/AppData/Local/Temp/446164281.pdf

 

Vince

[mbrun]

I ordered a 60 piece sample pack containing a wide variety sizes to use in the shack. I will report back my findings.

 

 

Michael

WRHS965

KE8PLM

[WRKC935]

Well, couple things probably going on. 

First is what cable were you using?  I am guessing that it was RG-58 or some other inexpensive cable.

I have seen this before with both poor cables and damaged cables / improperly installed connectors on cables where the shield was failing at the connection point.

It would be interesting to see what the cable loss on the cable you were using was. 

Cables are a mixed bag.  As are connectors.  A good quality phase stabilized lab grade test cable that is 6 feet long will cost over 200 bucks.

And the cable due to the connectors are rated for a certain number of uses as the connectors wear and begin to leak RF, and the captive center pins begin to loose their springiness and fail to conduct as well.

Mind you this is stuff that is used in the microwave engineering arena's and not even considered at the two-way LMR level of work.  But professional two-way guys even pay attention to their cables when dealing with 700/800 stuff.

I personally have seen overused cables cause radios to fail tests and alignments that were addressed by simply replacing a worn out cable.

 

If you were getting enough signal to hear it 1/8 of a mile away, you have a bad cable.  There is no other logical explanation unless the radio you were using had an internal antenna, or the antenna design was such that the antenna connectors outer body was the live connection.  Newer better quality radios use a SMA or BNC connector for the antenna.  Some of the old stuff like Motorola SP50's and others the antenna had no center conductor and fed the RF to the threaded part of the antenna.  This of course would turn the cable shield, dummy load body into an antenna. 

 

 

 

second was the quality of the dummy load.

Again, there are a number of grades of test equipment.  The old 'cantenna' dummy loads that were designed for ham HF use were a poor quality load and were designed for HF only use.

A good lab grade load is going to have an N connector or other good quality connector on it.  Be mounted with a flange mount and have a single non-inductive 50 ohm resistor in it.  Others, not so much.

[mbrun]

Here is the Dummy Load that was used.

 

https://www.amazon.com/gp/product/B07MKSVCYX/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

 

 

Here is the radio that was used.

 

 

 

Here is the cable that was used.

 

http://www.randl.com/shop/catalog/product_info.php?products_id=67753&osCsid=dkuldi6ak5c9bciv42djjoui56

 

 

 

One thing that occurs to me is that I could/can eliminate the cable from the equation all together by merely attaching the radio directly to the dummy load. I have all the necessary adapters. Just need to make sure zero stress is placed on the radio and connector, which is why the cable was used in the first place.

 

 

Michael

WRHS965

KE8PLM

 

Edit - Add two photos

[mbrun]

Alright, so I eliminated the 3’ cable and connected the radio to the dummy load using just an SMA to UHF adapter. I performed another test. This time I experienced full quieting reception out to about 300’, reliable reception with variable noise out to about 600’ and the complete loss of reception at about 975’. So it would seem the cable is definitely a contributor (perhaps 6dB more), but not the only one. That leaves us only two components. The radio itself and the dummy load. It would seem therefore it would take a faraday cage around one and then the other to determine which of these two ends up being the bigger radiator.

 

One significant take away from this exercise is the knowledge that just because one is using a dummy load it should not be assumed that one is not transmitting a signal, but instead only that ones transmission range is being substantially reduced compared to a purposeful antenna. So, if a dummy load were being used say in a large apartment building or condo complex, the radio signal may still very well permeate the entire complex or even beyond.

 

Also, as has been reported by me in other testing I have done, I experience very high propagation loss around me. Those without the dense stands of trees and other obstacles I have may very well see their signal travel much further.

 

Thanks for all your input.

 

 

Michael

WRHS965

KE8PLM

[tweiss3]

A few days late to this thread, but I would put money on the HT being the radiator. The HD is designed to be the counterpoise/ground plane for the rubber ducky, and therefore would probably radiate more so than the dummy load.

 

I would be interested to see this test done with a mobile unit, where it's designed with shielding to direct all RF to the coax.

[mbrun]

A few days late to this thread, but I would put money on the HT being the radiator. The HD is designed to be the counterpoise/ground plane for the rubber ducky, and therefore would probably radiate more so than the dummy load.

 

I would be interested to see this test done with a mobile unit, where it's designed with shielding to direct all RF to the coax.

You very well could be right. I think it would be a fair comparison if only the mobile rig could be set to the same 1/2 watt output power I am using on the HT.

 

 

Michael

WRHS965

KE8PLM

[tweiss3]

You very well could be right. I think it would be a fair comparison if only the mobile rig could be set to the same 1/2 watt output power I am using on the HT.

 

 

Michael

WRHS965

KE8PLM

 

Agreed. Have an 817/818 to test it with?

[mbrun]

Agreed. Have an 817/818 to test it with?

Sorry, not familiar with the numbers you reference. Could you clarify?

 

 

Michael

WRHS965

KE8PLM

[tweiss3]

Yaesu FT-818 or FT-817, will do down to 1/2watt and has a standard 259 plug on the back.

[axorlov]

The FT-817 has exactly the same behavior as HT on VHF and UHF - meaning body being a part of the radiator. It's not a radio problem/feature, it is rather the attribute of the wavelength. On HF the diminutive body of 817 does not contribute anything meaningful to the electromagnetic field, but on UHF the linear dimensions make it very close to 1/4 wavelength, thus it will radiate, and a lot.

 

However, the original post was about low-power transmission, not about how to judge the radiation from the dummy load.

[tweiss3]

The FT-817 has exactly the same behavior as HT on VHF and UHF - meaning body being a part of the radiator. It's not a radio problem/feature, it is rather the attribute of the wavelength. On HF the diminutive body of 817 does not contribute anything meaningful to the electromagnetic field, but on UHF the linear dimensions make it very close to 1/4 wavelength, thus it will radiate, and a lot.

 

However, the original post was about low-power transmission, not about how to judge the radiation from the dummy load.

 

Does it when you switch it to the back 259 port? I figured it did on the front BNC because that is designed for the rubber duck in the box.

[OldRadioGuy]

Cable Radiation

 

I just want to point something out.

 

Eliminating the cable reduced the radiation.  But does this mean it was bad cable? No.

The shielding of this cable is probably just fine.

You could replace this cable with the finest quality and it would still radiate just the same.

This is because it's common mode radiation.

The whole cable is a sort of antenna.

 

Only a ferrite or better grounding system would reduce the common mode radiation of the cable.

Don't go out and buy a more expensive cable to solve this problem.

 

Vince

[mbrun]

Yaesu FT-818 or FT-817, will do down to 1/2watt and has a standard 259 plug on the back.

 

Thanks. No, I don’t have either of those to experiment with.

 

 

Michael

WRHS965

KE8PLM

[axorlov]

Does it when you switch it to the back 259 port? I figured it did on the front BNC because that is designed for the rubber duck in the box.

Of course, the back connector and the front BNC connector are bolted to the same aluminum body of the FT-817/818. The ground bus inside the radio electrically bonded with body and case in several places.

 

I can only come up with the one sure way to isolate the body of HT or any other radio: put in Faraday cage NOT electrically connected to the body, but providing very low impedance and very short path to the ground. Something along "wrap your HT with kitchen foil, put inside the zip log and bury in the wet sand on the ocean beach". Also, have a very high-impedance RF choke on the coaxial. But how are you going to press PTT and speak/listen? The remote mic cable will radiate better than HT body, it's longer! (look up so called Tiger Tail) And I will repeat, this is the property of the short wavelength. The shorter, the easier for RF to escape through ground buses, connectors, cables, etc. The only solution is to wrap yourself together with your HT and have yourself buried with the radio in the wet sand. You would need a snorkeling pipe. Make sure your will is current and signed by two witnesses. And report the results! I'm genuinely interested.

[axorlov]

After giving it some thought, it may be a valid experiment. HT (or FT-817) is inside the zip log, wrapped with kitchen foil, on the wet sand or swamp ground. Make sure there is absolutely no electrical connection between radio, or cable shield and the foil. MASSIVE choke on the coax, inside the same Faraday cage. Operator 1 is next to the radio pressing PTT through the foil, Operator 2 is measuring field strengths with whatever method available.