Newbie RF frontend protection Qs

Useful information regarding antennas for SDR products.
Roger
Posts: 100
Joined: Fri Dec 29, 2017 7:59 pm

Re: Newbie RF frontend protection Qs

Postby Roger » Thu May 24, 2018 2:28 am

The diodes will clamp the RF input to less than 1 volt zero to peak (app. +10 dBm) unless they are damaged due to excess current. The 1n4148 is rated for 300 ma continuous maximum forward current and a repetitive peak forward current of 500 ma with surge up to 2 amps. The tungsten bulb acts like a fuse because it is only rated at 150 ma and will burn out at currents two to three times that amount. An incandescent tungsten bulb has a cold to hot resistance ratio of 10-15 to 1. When cold the bulb will have a resistance of less than 4 ohms and when operating at the rated current (150 ma) the 6V bulb will have a hot resistance of 40 ohms. If a strong received signal of S9+60 dB (-13 dBm) is applied to the input this will be 50 millivolts RMS, the bulb resistance will be less than 4 ohms and the diodes will not be clipping the signal (however they will generate weak intermodulation products). If a very high +30 dBm signal (7 volts RMS) is applied to the input the bulb will have 6.3 volts RMS across it and be lit with an on resistance of 40 ohms. The diodes will now be clamping the input to the receiver at +10 dBm and protecting it (RSP's are rated at 0 dBm continuous, +10dBm intermittent). If the voltage were to increase to 20 volts RMS (39 dBm) the current would be around 500 ma. and the bulb will quickly burn out and no signal will reach the receiver.

The bulb in this design is the weak link in the chain and if it burns out before the maximum current and power handling of the diodes is exceeded the receiver front end is protected.

On a side note the RSP1 used BAV99 surface mount diodes and because of their size they cannot dissipate much heat. I do not know if they have used this same device in the RSP1A, RSP2 and RSPduo. In any event the small physical size of thee devices means you need an external RF protector. The diodes are mainly there for ESD protection and not strong RF signals.

I would also add a GDT device for static and distant lightning protection if I were building a front end protection device.

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vk7jj
Posts: 126
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Re: Newbie RF frontend protection Qs

Postby vk7jj » Thu May 24, 2018 2:47 am

If 7 volts is applied to the input (+27 dBm) the bulb will have 6 volts across it and be lit with an on resistance of 40 ohms.


Why would there be just 7 volts at the input? Why wouldn't 7,000 volts be any more likely or unlikely?

If we accept that the resistance is 40 ohms at 150ms what would it be at 5ms?

Cheers, Phil

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9a4db
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Joined: Sun Jan 24, 2016 6:44 pm

Re: Newbie RF frontend protection Qs

Postby 9a4db » Thu May 24, 2018 5:20 pm

This is my solution, protection with distribution of signal to two RX, designed initially for Sony ICF 2001D RX...
D1-D4 = > 1N4148
Discharge element, Bourns 2057-07 Series Light Duty 2-Electrode Miniature Gas Discharge Tube (75 V)
Bulb 12 V 250 mA

Image

Picture taken before Bourns 2057-07 was installed
Image

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vk7jj
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Re: Newbie RF frontend protection Qs

Postby vk7jj » Fri Jun 01, 2018 2:19 am

D1-D4 = > 1N4148


Hi Djani,

I use the same diodes and this whole protection issue has bothered me for years.

I'm guessing your use of series diodes raises the issue of the effects of clipping where a single diode is used and the received signal is strong enough to be desirable but not so strong as to warrant protection, if that's not an oxymoron. I'm too far away from anything remotely strong enough to be in that category but I finally got off my bum and made a few quick measurements of induced power in one antenna while transmitting on an adjacent antenna.

Antennas: two 160m diameter delta loops, their fed corners are at their closest points to the radio shack and are 15 meters apart.

Transmitter on antenna #1: 50 watts

Measurements were made on antenna #2 of the peak-to-peak volts as viewed on a 0-30MHz oscilloscope terminated with a 1k resistor:

1.8MHz -> 63 volts
3.6MHz -> 60 volts
7.1MHz -> 14 volts
10.1MHz -> 16 volts
14.2MHz -> 12 volts
18MHz -> 16 volts
21.2MHz -> 16 volts
24MHz -> 24 volts
28.4MHz -> 6 volts

I used 50 watts as a conservative starting point, as we know many hams use much more than that.

Like most hams I have a bunch of other antennas on 6M, 2M and 70cm all on the same tower as antenna#1 is terminated at, with two Yagis only a meter or two away depending on where they are rotated. The 30MHz CRO meant I couldn't check 6m which is where I think my most risky situation might arise.

As per Roger's use of a filament, is it the unstated consensus that the diodes will outlast the filament, ie. they will clamp the voltage long enough for the filament to rise to it's burn out point and act as a fuse?

My unstated concern is that with (say) a 100 watts 6m signal on antenna #1, the random length of my feed line in respect to a 6m wavelength on antenna #2 may via impedance transformation result in a significantly higher voltage with a very fast rise time at the receiver than my lower frequency measurements reveal.

Also the terminating impedance I used was a 1k ohm resistive load, in contrast people may well be using a 1:9 balun connected to the high impedance input of an RSP2 resulting in significantly higher voltages at the RSP.

Regards,

Phil

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9a4db
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Joined: Sun Jan 24, 2016 6:44 pm

Re: Newbie RF frontend protection Qs

Postby 9a4db » Sun Jun 03, 2018 7:13 pm

Hi Phil,
The only reason of double in serial diodes use, is to avoid intermodulation during
the evenings when HF signals are stronger.

For the rest, please look on my schematic. Let's imagine few possibilities:
1. Diodes are ground for all +1,4 or -1,4V or more on input... if is to much current - than diodes are history ;)
than....
2. pure static DC is grounded permanently through primary side of splitting RF transformer
3. pulses also going to ground thru the same path, if strong enough bulb filament may rise the resistance,
if pulse level is strong enough (over 75 V) than gas discharge tube is ground too. All that must result in bulb filament burn out.
Result antenna disconnected.

The only weak point of my solution is: if on one of exits some TRX is connected and PTT keyed, than the
other side is smoke :D

73
Djani

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vk7jj
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Re: Newbie RF frontend protection Qs

Postby vk7jj » Mon Jun 04, 2018 9:57 am

All understood Djani and thanks for your comments.

The issue I've tried to resolve from the outset has been delay. As per my first post filament delay is substantial being typically 150 ms as per the link I quoted. So is the response time of gas discharge tubes.

As per Wikipedia

"GDTs take a relatively long time to trigger, permitting a higher voltage spike to pass through before the GDT conducts significant current. It is not uncommon for a GDT to let through pulses of 500 V or more of 100 ns in duration. In some cases, additional protective components are necessary to prevent damage to a protected load, caused by high-speed let-through voltage which occurs before the GDT begins to operate."

The only hard data I could easily find to quantify that is in this pdf https://www.researchgate.net/publicatio ... arge_Tubes

The graph shows a Siemens device that takes 10^4 ms before any current is conducted and 10^6ms before maximum current flow is reached. I'd love to think a 1N4148 diode will last long enough for a filament and or a gas tube to kick into effect but "hope" is a poor strategy; by comparison the dot length of 50 baud CW is in the order of 20 ms and the rise time is 5ms or so.

The 1N4148 specs show a reverse breakdown voltage of only 100V and a maximum forward current of around 300mA so while they are speedy they aren't specially robust in the face of a casual transmission on a closely coupled adjacent antenna, perhaps we should consider parallel arrays of diodes.

I would like to find a well documented article that shut my mind up!

Regards and thanks for the discussion, Phil.

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9a4db
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Re: Newbie RF frontend protection Qs

Postby 9a4db » Mon Jun 04, 2018 12:19 pm

Dear Phil,
it was pleasure to read and write some comments on this important topic.
My opinion is that your concerns about response time are just right.
I am using above device since 2015. on my summer stormy island location, so far no damage behind ;)
or within device.

GL & 73
Djani

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Roger
Posts: 100
Joined: Fri Dec 29, 2017 7:59 pm

Re: Newbie RF frontend protection Qs

Postby Roger » Mon Jun 04, 2018 4:55 pm

How has the N8VB FEP circuit I posted earlier performed in HF contest stations with kilowatt transmitters nearby?

From this link... http://valentfx.com/vanilla/discussion/ ... ng-antenna

"Here is some advice from Gunnar, SM3SGP, who manages the Kiwi beta test unit at the SK3W contest station in Sweden.
"I have made the circuit described by N8VB on the QS1R Wiki pages. It consists of a small light bulb, and two diodes. I think I have changed that light bulb once, in many years of operation. We are running Multi TX Kilowatt output, but I have never had any failure of the QS1R. http://www.srl-llc.com/qs1r_latest/Docu ... rxprot.pdf"

From this link.... https://www.hamradiosecrets.com/ham-rad ... s.html#FEP

"I tested the FEP's sturdiness and capacity to shunt strong RF signals to ground ... in the most violent way at my disposal!
Using a UHF "T" adaptor (SO-239 connectors) I tapped the transmission line between my IC-7200 XCVR and its antenna.
Then I connected the FEP to the tap and a 50 Ohm dummy load to the receiver end of the FEP.
I ran three tests at the following power output levels: 25 watts, 50 watts and 100 watts.

The FEP survived 25 watts and 50 watts, effectively clamping down the excessive RF signals to ground. The GF550 lamp fuse finally burned open at the 100-watt level. The FEP worked as Phil (N8VB) said it would!

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vk7jj
Posts: 126
Joined: Thu Mar 02, 2017 9:56 am

vk7jj

Postby vk7jj » Tue Jun 05, 2018 5:18 am

Hi Roger.

How has the N8VB FEP circuit I posted earlier performed in HF contest stations with kilowatt transmitters nearby?


"How" is the question I hoped you might be able to answer.

If he only thinks one bulb has burned out in many years of operation, does it look they work as as a fuse?
There's no mention of diodes ever being fried, does that mean diodes alone are good enough?
Or, do the diodes conduct long enough for the globe to provide 40 ohms resistance without blowing out and can just 40 ohms save the day?

"The Kiwi itself uses TVS diodes across the RF/HF inputs"
and
"We have not done any limit testing of the inputs"


TVS diodes are designed for the job, from Wikipedia"

"...made and tested to handle very large peak currents. The 1.5KE series allows 1500 W of peak power, for a short time."

Perhaps the Kiwi's built in TVS diodes alone save them and it had nothing to do with any external circuitry?
Perhaps it was both? Perhaps it was a lucky combination of the antenna spacing, the resonant length of the feed lines and the frequencies in use?

Regarding your second link, the FEP, don't you find a credibility problem with the article?

He says his 100 watts test blew the globe and in the same breath he says he's happily transmitting quote "a full 100 watts on CW"!!

Anyway his tests relate only to his own antenna, feed line and frequencies in use.

Any nasty incoming signal voltages and currents at his receiver will swing between current maximum/voltage minimum and current minimum/voltage maximum every 1/4 wavelength of feed line and antenna at the respective frequency. Just like the voltages at the ends of a centre fed resonant dipole that can typically be in the order of 6,000 volts.

ref: https://ham.stackexchange.com/questions ... le-antenna

His test failed to take into account the protection his radio already has built in, perhaps the clamping devices in his radio are what blew the globe.

Herein is my problem, restated for clarity

1. Globe filaments and gas discharge devices "do not exist" during their slow turn on time. None of my commercial radios have them in them. Are there any commercial devices being offered to the ham market that use them? So, do they make a difference and if so how? I have no answer.

2. Diodes are widely used, with the right combination of resistance and capacitance may be that is all that is required. They've worked fine for me so far but I take care by switching my SDR line's coax switch when I'm using the 6m beam, for example. That's a pain and why I'm trying to find a better solution.

Best hopes this discussion is entertaining someone ;-)

73 Phil VK7JJ

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Roger
Posts: 100
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Re: Newbie RF frontend protection Qs

Postby Roger » Tue Jun 05, 2018 7:28 pm

vk7jj wrote:Sadly, filament globes take ages to achieve maximum resistance and even when they do the resistance value is not even a tiny speed bump from the point of view of the value and time of fast rising voltages as per not so close lightning or PTT keying of transceivers into nearby antennas.

https://www.picotech.com/library/results/lamp

Comparing the filament's 150ms turn on time with with typical radio keying times, a quick search found: "The ARRL recommends a 5 mS rise and 5 mS fall time for CW"

Regards,

Phil


Phil - Incandescent bulbs take far less than 150 milleseconds to achieve maximum resistance. In 5 milliseconds
they are at 1/2 maximum resistance and at 10 milliseconds have reached about 80% of maximum resistance.

Source 1 - https://vcclite.com/wp-content/uploads/ ... -paper.pdf

bulb.PNG
bulb.PNG (28.87 KiB) Viewed 844 times


Source 2: - https://www.allegromicro.com/~/media/Fi ... ECEEE9A2AF

Tungsten-filament incandescent lamps exhibit a very-high
positive temperature coefficient of resistance with the cold filament
resistance being approximately 10% of the hot filament resistance.
When an incandescent lamp is initially turned on, the cold filament
is at minimum resistance and will normally allow a 10x to 12x peak
current. Within 3 to 5 ms the current falls to approximately 2x the
hot current

Source 3: http://www.tlt.co.jp/tlt/english/produc ... g/info.htm

bulb2.png
bulb2.png (13.49 KiB) Viewed 844 times

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