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[Articles Home]  [Add Article]  

Rare Pedestrian Mobile Contact with UK

Paul Signorelli (W0RW) on July 26, 2005
View comments about this article!

This rare contact took place between Monument, Colorado, and Essex, UK. It was the annual gathering of HFPack'ers at the PPRAA Swapfest in Colorado.

Jake, N0LX/Bike Mobile, said he just heard Tom, G0SBW/Pedestrian Mobile, on 18157.5 kHz. I thought it was a joke, the bands seemed dead for the past week. Then Gary (N5ANF/pm) heard him. He was really there! I tried to contact him with my 50W backpack radio and 10-foot whip but he could not hear me.

The Swapfest was being held at the local high school and they had two baseball fields nearby, so I headed for the field. There was no game being played on the field at the time. I heard Tom calling and walked to the focus of the baseball backstop fence, near home plate.

The backstop was about 45 feet high and 'pointed' northeast. Tom heard my call and responded. It was weak but readable. I saw another 30-foot backstop that was 'pointed' in a slightly different direction in a different field and walked over to it; it worked much better.

Tom was able to understand that I was on a Cricket Field. The backstop was a truncated corner reflector and the focus probably gave me +10 dB gain, 500W ERP. (It was kind of like standing in front of the FPS-50 in Clear, Alaska).

Gary, N5ANF, was standing there too and tried to make contact with his QRP pedestrian mobile with a half wave antenna but Tom could just barely hear him in the noise. This baseball backstop is the ultimate HF Pack antenna accessory. The groundskeeper did want to know what was going on.

Tom, G0SBW, added the following comments:
After an afternoon spent with my XYL visiting local gardens I got home in time to dash out to try and catch the last of the 1630Z HFPack net whilst operating pedestrian mobile. I tried 17m first. This looked promising, as I had heard at my QTH a Mali station, TZ9A some 2000 miles away, running a big pile up on 18160.

I put out a call on 18157.5 and was very surprised to be answered by MI0MRV, Marv in Northern Ireland. He was followed by GM7JOM/M in Scotland who in turn was followed by G7JVG and M3OCE both in the NE of England. This was a new experience for me to work stations 200 to 350 miles away in the UK on 17m. Normally, on this band, I only work UK stations close by on ground wave. Despite hearing the Mali station no one else came back to my calls.

Then, at about 1750Z, I heard "Romeo Whisky", "Mobile" and other interesting words just above the noise level. Eventually I heard my call sign and "Whisky Zero Romeo Whisky.” A difficult QSO with Paul followed. At first I did not understand why he was on a sports field -- hmmm must be a very talented HFPacker -- operating /PM AND playing baseball at the same time! Time to get out my cricket bat, add a coil and some wire and try and add to my DXCC score. Paul was in Colorado some 4500 miles away.

For this Pedestrian Mobile contact, G0SBW, used a Yaesu 817, Tokyo HL-50B amplifier and LDG Z11 autotuner. 50w of SSB to a 12 foot whip antenna. He was in Essex, UK. Thanks to Mark, KI0PF, who took some great pictures of the operation. For more information about pedestrian mobile operation see www.HFPack.com.

Paul, W0RW

Member Comments:
This article has expired. No more comments may be added.
 
Rare Pedestrian Moblile Contact with UK  
by KA4KOE on July 26, 2005 Mail this to a friend!
Good deal Paul. Its exciting. I remember the day this past March that my PRC2000 came in. I worked Belgium with 20 watts from the backyard on a 9' whip.

FUN FUN FUN!!

Good chatting with you the other day!

PHILIP
 
RE: Rare Pedestrian Moblile Contact with UK  
by K0BG on July 26, 2005 Mail this to a friend!
Hi Paul, its been a while.

This just goes to show you how little radiated power it takes to operate across the pond. I don't know what date you made your contacts, but about 6 weeks ago, 17 meters sounded like 20 meters at its height a couple of cycles ago. If you tried hard enough, you could have worked DXCC in a few hours, especially on CW. Great fun, huh?

Alan, KØBG
www.k0bg.com
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 26, 2005 Mail this to a friend!
.
Should you be standing 1/4 wave (12.9 feet) in front of your baseball stopper, your effective radiated power would be approximately 5 dB and that amounts to a gain of 3.16 times your 50 watts which equals 150 watts of ERP.

.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by WA6BFH on July 26, 2005 Mail this to a friend!

A good and compelling article! Even HF can yield some real fun!

I would agree more closely with TH’s thoughts on the gain you might achieve from your back-stop. I was going to say 3 to 6 dBd. Now W6TH loves to play with these mystical magical Isotropic radiators, so my gain figure may be optimistic, uh, or is that realistic!

Its all good though! I’m also guessing that your pack radio might do better if it was green!
 
Rare Pedestrian Moblile Contact with UK  
by YI9VCQ on July 26, 2005 Mail this to a friend!
Totally cool! Nice article.

73,

Korey
YI9VCQ/KA5VCQ
Al Asad, Iraq
 
Rare Pedestrian Moblile Contact with UK  
by WA1RNE on July 26, 2005 Mail this to a friend!
Sounds like fun. Using the chain link back-stop is a like having a 2 element "Pedestrian Beam."


Not to rain on your parade, but have you and other pedestrian mobile operators considered the amount of RF exposure you are subjecting yourselves to?


Going by the University of Texas RF Safety calculator:


Using your "Pedestrian 2 element Back-Stop Beam" and assuming about 5 dB over a dipole, here's the RF Exposure calculations:


Average Power at the Antenna 6.700 watts
Antenna Gain in dBi 5.00 dBi
Distance to the Area of Interest: 1.00 foot
Frequency of Operation 18.000 MHz
Are Ground Reflections Calculated? Yes


Estimated RF Power Density:

** Controlled Environment: 4.6460 mw/cm2

Distance to Compliance From Center of Antenna: 1.34 feet

Maximum Permissible Exposure (MPE) 2.78 mw/cm2


** Uncontrolled Environment: 0.56 mw/cm2

Distance to Compliance From Center of Antenna: 2.94 feet


>> Does the Area of Interest Appear to be in Compliance? NO (both environments)


I used 6.7 watts average power for the calculation. This is based upon using SSB with 20% average power output at a 67% duty cycle - 10 minutes on, 10 minutes off for a total of 30 minutes.


Without the backstop, just using the whip, the Estimated RF Power Density drops almost 50% to 2.4383 mw/cm2, but it's still close to the controlled environement exposure limit. The non-controlled limit is still 0.56 mw/cm.


Personally, I would use the NON-controlled specification as the exposure limit, as this is what the general public would be exposed to, like your neighbors - and because you're walking around with a whip very close to your body or others, versus an antenna that's 30 feet away in a tree or on top of the house.


Again, I'm not trying to create a negative spin on this article - I think your idea is great - but RF Exposure limits are something to be taken seriously and am genuinely concerned.

Maybe you or someone else have made some other calculations and came up with different results?


73, Chris
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 26, 2005 Mail this to a friend!
.

Actually John, this dB information is from when I was using my Calibrated sight and using my Polar recorder.

For a 10 foot whip, Paul W0RW should try to find a way to phase another 10 footer and then can reduce his power by 3 dB or to 25 watts to conserve the battery power.

W6TH
.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 26, 2005 Mail this to a friend!
.

Chris

I have been around the Press Wireless transmitters for several years running 40 Kilowatts of power output with rhombic antennas and never had a problem with RF. I also was in with one rig putting out one million watts of power and never had any ill effects, also using rhombic antennas.

I was still producing babies at the age of 80, so the proof is in the pudding.

BTW, the rhombic antennas had a forward gain of 20 dB and were the terminated rhonbics.

73, W6TH
.:
 
Rare Pedestrian Moblile Contact with UK  
by W0RW on July 26, 2005 Mail this to a friend!
You can see a picture of the Backstop Antenna by clicking on the link at the top of the article:
"Paul Signorelli (W0RW)"
There are more pictures at
http://photos/groups.yahoo.com/group/milpack/lst
then go to the "W0RW Ped" Folder
These Backstop Antenna photos were taken by Mark, KI0PF

The contact date was 11 June 2005, 1750z, on 18157.5 KHz. W0RW/pm to G0SBW/pm.
It was a fun contact.
Paul w0rw
 
RE: Rare Pedestrian Moblile Contact with UK  
by WA1RNE on July 26, 2005 Mail this to a friend!
Vito;


Babies at 80?? You're a better man than I'll ever be.


What's your secret???



73, The Envious One
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 26, 2005 Mail this to a friend!
.
Chris

My secret you ask?... Having a beautiful young natural blonde wife at 20 years younger than I.

Also standing in front of my wave guide at maximum power.

73, W6TH
.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by K3GAU on July 26, 2005 Mail this to a friend!
Interesting comments on RF exposure however some of you are forgeting that the gain is at a 'far field' point where the antenna is aimed or focused. Being located IN the antenna the 'gain' is not that much.

Some years ago I had the opportunity to visit with Sam Harris (W1FZJ) at the giant 1000 ft. radio telescope in Arecibo, PR. We were talking with him about the tremendous ERP power it generated (100's of megawatts) He said it was fine to walk around the surface of the dish while the transmitter was operating and they typically did so. He said the the power level per sq unit of surface area was actually quite low even when generating those ERPs. If however you were to fly overhead at some distance above the dish (in the far field) you could be toast. Just something to think about.

Dave
 
RE: Rare Pedestrian Moblile Contact with UK  
by AK2B on July 26, 2005 Mail this to a friend!
W6TH wrote>
"I was still producing babies at the age of 80, so the proof is in the pudding.
"
You go boy!
Were their blood types RF positive? :-)

Tom
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 26, 2005 Mail this to a friend!
.

Paul has come up with a very useful means to go PM. Think this over and may bring more to try PM.

This is a nonisotropic method to have ham radio fun and what paul has done was to make properties that differ according to the direction of measurement, also to differ the radiation pattern and the navigational direction by moving around.

Should you have a problem trying to understand this, you may contact John WA6BFH for a further explanation.

73, W6TH... The Dot and Dash Scientist.
.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 26, 2005 Mail this to a friend!
.

AK2B

Dunno, gotta take a DNA.

.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 26, 2005 Mail this to a friend!
.

.
Paul W0RW

I forgot to mention that not only did you improve your signal with the ERP of 5 dB, but you also improved your receiving ability by the 5 dB which is equal to one "S" unit.

That was good thinking on your part to make the choice of finding a better place for a stronger signal.

We did that years ago going mobile on 10 meters, driving to a spot for a better signal.

I can imagine the happiness and enjoyment that overcome you.

73, W6TH.
.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by WA1RNE on July 26, 2005 Mail this to a friend!
The consideration of far fields is a good point; there is also a cross-over point between near and far fields which is closer, but let's not get go down that road.


In general, I'm just not fond of placing my head near anything that could be radiating 6-7 watts of RF (average) for a long length of time, to as much as 50 watts CW or PEP.


As I was writing this, I stopped, leaned over my shoulder and unfortunately watched pitcher Matt Clement of the Red Sox get hit in the head with a line drive.

Everybody say a quick prayer for him, it was a nasty shot.


You gotta protect your head.....

 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 26, 2005 Mail this to a friend!
.

RF exposure is not my problem or worry.

My worry is to ordinary life and that is getting killed by a jealous husband.

.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by KC9AGG on July 26, 2005 Mail this to a friend!
The backstop was a truncated corner reflector and the focus probably gave me +10 dB gain, 500W ERP. (It was kind of like standing in front of the FPS-50 in Clear, Alaska).

it was a tuned reflector???huh???
 
RE: Rare Pedestrian Moblile Contact with UK  
by KL7IPV on July 26, 2005 Mail this to a friend!
Way to go, Paul. Aint PM fun? Maybe if you get enough RF yourself, you can brag when you are 80! In the meantime, keep having fun.
73,
Frank
KL7IPV
 
Rare Pedestrian Moblile Contact with UK  
by W8KQE on July 27, 2005 Mail this to a friend!
Interesting article and nice work! I dabble in 'HF Pack' type activity once in a while myself just to keep the hobby interesting and challenging after so many years. My favorite 'HF Pack' QRP contacts occured when 10 meters was in great shape a few years ago using a 5 watt 'AA cell powered' 10m SSB walkie-talkie (Spitfire 454). I worked a few Europeans during a contest using a base-loaded 3 foot whip and a 1/2 wave counterpoise alligator-clipped to the body of the walkie-talkie and dragging on the ground behind me. All while standing in my driveway! It still took hundreds of calls just to work them, but the thrill of finally working that DX with a 'lowly' handheld QRP unit was worth it!

 
RE: Rare Pedestrian Moblile Contact with UK  
by WA6BFH on July 27, 2005 Mail this to a friend!

<The consideration of far fields is a good point; there is also a cross-over point between near and far fields which is closer, but let's not get go down that road. >

Does the above comment have something to do with my Harry Potter story?

Seriously, it is good to hear the comments about 10 Meters! Guys, you don’t have to wait for the sunspot peak to work 10 Meters though! The E-layer works fine all year every year!

Put up a re-tuned CB Ground Plane, and monitor your favorite frequency of activity (guys with pack radios could fabricate a nice Quarter-wave Ground Plane with a wire radial kit). I believe Mosley makes a nice 5/8 wavelength 10 Meter Ground Plane antenna. If you really want to “do it to the 9’s”, put up a pair of vertically polarized Yagi’s. But the Ground Plane still has the virtue of always being pointed in the correct direction!

You will be amazed how often 10 Meters opens to E-skip! You then may desire to follow the MUF up to 50 MHz with a similar antenna arrangement. The MUF won’t make it up to 50 MHz nearly as often as it will 28 but, it’s all good!


 
RE: Rare Pedestrian Moblile Contact with UK  
by WA5ZNU on July 27, 2005 Mail this to a friend!
WA1RNE wrote:
>Maybe you or someone else have made some other calculations and came up with different results?

How about this calculation?

With a 5W 2 Meter HT on FM at 50% talk time, the same Texas calculator shows you need to keep it 10 inches away and not come closer than about 2 feet to anyone else. That's assuming your rubber duck is a 0dBi radiator:

Average Power at the Antenna 2.500 watts
Antenna Gain in dBi 0.00 dBi
Frequency of Operation 145.000 MHz
Distance to Compliance From Center of Antenna
Controlled: 0.79 feet
Uncontrolled: 1.71 feet

So let's try a 17" whip, a generic $30 HT antenna, which Comet reports at 2.15dBi. That gives us
Controlled: 1.00 feet
Uncontrolled: 2.17 feet

Now, if I assume a 6dB gain from a corner reflector (much easier to obtain at VHF -- maybe just the corner of an aluminum building) we get

Controlled: 1.94 feet
Uncontrolled: 4.28 feet

So, if you have trouble getting into the repeater and move around until you get a reflection off a nearby office building, according to these assumptions and this UTARC RF safety calculator, you had better hold your HT 2 feet away from your body (that includes your hand, by the way), and draw a 4ft chalk circle around yourself.

Note that these distances are very close to the 3ft or so you calculated for Paul's HF signal strength, and are based on the same kinds of assumptions (antenna really has gain, gain really is pointed in your direction, etc.)

Summary: I do not believe that the assumptions of signal strength and patterns and the imprecise modeling justify the levels of concern expressed here, and that at best, we can use such tools to give order-of-magnitude measures only, especially when considering power in the 0-20W range. (In fact, when the FCC approves cell phones, they use SAR data calculated using absorption data using real devices, RF chambers, robot arms, etc.)

Leigh / WA5ZNU
 
Rare Pedestrian Moblile Contact with UK  
by G0GQK on July 27, 2005 Mail this to a friend!
The contact between G0SBW and W0RW was quite an achievement when one considers they were both pedestrian operators. It is not surprising, as Tom points out, that this was a difficult contact, the distance between them being 4,500 miles.

I rather think the suggestion of 150 watts ERP mentioned by one correspondent is perhaps a teenzy weenzy bit optimistic ! Operating on 18.157 Mhz with 50 watts output into a 10 ft long vertical antenna, the efficiency on this band with a vertical is roughly about 25%, so the radiated power was probably somewhere about 13.5 watts, with some help from the metal backstop posts. Amazing !
 
RE: Rare Pedestrian Moblile Contact with UK  
by WA6BFH on July 27, 2005 Mail this to a friend!

ZNU, that’s a nice piece of non-science!

A corner reflector cannot be modeled by “the corner of a metal building”. Not unless you are inside that building!

You might better conceptualize a “corner reflector” kinda sorta analogous to a parabolic dish. The radiating element needs to be at the focal point of that dish -- or inside the two planes of the “corner reflector”.
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 27, 2005 Mail this to a friend!
.

G0GQK

Thanks for your analysis of being subjected to a methodology involving algebra or other methods of mathematics.

Does that mean the G5RV antenna is very inefficient; not producing the intended result by only being 102 feet on 80 meters, whereby it should have been 133 feet?

.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by WA5ZNU on July 27, 2005 Mail this to a friend!
WA6BFH, yes, but I was envisioning a concave corner, not a convex one! I think that the distannces and scales are close enough to making a reflector for VHF, at least as well as a baseball fence does for 18MHz, which was the point, hi hi.

Here's a picture I found on the web showing the kind of inner corner I meant: http://tinyurl.com/budpj This building has them, a giant one in front right, but smaller ones in the back.
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 27, 2005 Mail this to a friend!
.

WA6BFH

My first use of a corner reflector was when I was a boy scout and was going for a merit badge for cooking. I made a corner reflector of two highly polished aluminum plates at 90 degrees and used them for baking my pancakes and frying two eggs and two slices of bacon, in the heat of the sun. No fires needed for this.

Needed one more badge for my Eagle Scout and that was bird study.

Suggest you make the design to reflect RF and also to cook yourself a meal.

73, W6TH
.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by N0TONE on July 27, 2005 Mail this to a friend!
Congratulations on making the QSO.

At the risk of raining on the parade, though, I must admit that I'm not enthralled at these things being "new". I am frequently surprised at how the present HF pedestrian backpack crowd seems to be putting more elaborate things on the air, but getting less results.

In the 1970s and 1980s, we had a very acive HF orgnisation known as "Bicycle Mobile Hams of America". Mainly we were using QRP homebrewed or kit-built radios. Two to five watts was the norm, probably nine out of ten of us used CW exclusively, but SSB was not unheard of. I had SSB capabilities, it is really rather difficult to manipulate a key whilst bicycle mobile. The typical antenna of the day was one of the Outbacker models, clamped to the "mountain rack" on the back of the vehicle. When I look at the "restrictions" that the HF Pack community puts on antennas to qualify as a legitimate "HF Pack" antenna, our bicycle antennas were well within the limits - certainly nobody I knew dragged a counterpoise!

With these radios and antennas, DX was the norm. Of course, most of our QSOs were with "home" stations, but that was principally because there weren't many other bicycle mobiles around, given the absence of commercially made gear for the purpose.

However, all of the bicycle mobile HF ops I cavorted with had at one time or another, crossed the pond to another bicycle mobile operator. It was not a daily event, but common enough that one would definitely experience it at least once per active month.

Why, all of a sudden, it is a rare thing? Have we bred poorer operators? Do today's operators have less knowledge of how to select the correct band?

We even had a few chaps work WAS or WAC exclusively with other BMH (Bicycle Mobile Hams). Likely the first of the gang was Yardley Beers W0JF, he was the first to do anything. I enjoyed cavorting with him, teasing him about being younger than I was. The good old days in Boulder.

I think everybody ought to try this. It should be easy, I am surprised at how much difficulty the present crowd of HF Pack ops are having doing these things.

AM
 
RE: Rare Pedestrian Moblile Contact with UK  
by WA6BFH on July 27, 2005 Mail this to a friend!

Vito, you and I should write an article for this page under ‘joint-publication’.

We have been around long enough, albeit I am your obvious junior, that we have learned a few things -- at least I think I have (a certain YL will tell you otherwise).

What’cha think? What subjects are these Hamster’s in need of hearing a couple of old farts talking about and describing -- in painfully elaborate detail?

73! de John
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 27, 2005 Mail this to a friend!
.
John,

#1 It must be a dream, an illution; something, such as a fantastic plan or desire, that causes an erroneous belief or perception.

Lets see: A ground plane, nope, 15 radials or less, nope, math projects, nope, how about dBi or dB? Nope, cw, nope. Nope none of these.

How about how I worked DXCC on 70 cm? Nope, let me sit on this for a day or so. No! Not that either.

Lets go back to #1 John.

#1 It must be a dream, an illution; something, such as a fantastic plan or desire, that causes an erroneous belief or perception.

73, W6TH
.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by W6TH on July 27, 2005 Mail this to a friend!
.

It is suppose to be illusion John. Don't know how that happened.

Vito
.:
 
RE: Rare Pedestrian Moblile Contact with UK  
by WA6BFH on July 28, 2005 Mail this to a friend!

Ok Vito! You think about it for awhile!

73! de John
PS
You weren’t ever a hippie were you?
 
RE: Rare Pedestrian Moblile Contact with UK  
by N0TONE on July 29, 2005 Mail this to a friend!
Vito, has anybody worked DXCC on 70cm? I know it's been done by a lot on six meters, and maybe someone on two meters, but 70cm?

What band/freq are you most commonly found on? I'm mainly on 40CW, circa 7-9pm your time, usu 7010, working DX. Depends on where I'm staying at the moment. Trying to sell some of my houses since I can't keep up with them any more.

Being some years your senior, I sense that I'm starting to fade fast, so you better tell me how to find you on the bands before I'm not around to do it. You've probably seen the signs of dementia in my postings...I'm not always cogent any more. Sometimes I know I'm not being cogent, at other times I don't even know I did something. Was hoping to see 100, but if I get there, I probably won't know I did it. Stinks, really. Maybe one of my kids will tell me about it when we meet up in the afterlife.

AM
 
Rare Pedestrian Mobile Contact with UK  
by W0RW on August 6, 2005 Mail this to a friend!

The purpose of the article was to encourage others to get out and work portable HF.

Here are some answers to some off topic questions:
1. The gain of a corner reflector per the ARRL VHF Handbook 1965, p.224, is 10 dB if you can get at least 1 wavelength on a side. The reflector is not a 'tuned' reflector. One thing i forgot to do was to retune my antenna tunner. The radiation resistance of the antenna changes as it is placed into the center
of the reflector.

2. RF Safety. You can input your radio's power and antenna into the University of Texas RF Safety calculator at
http://n5xu.ae.utexas.edu/rfsafety/
If i run 50W, with a CW duty cycle of 20%, my average power is 10W.
If you use zero dBi on 14 MHz, with no ground reflections it gives a
safe controlled area distance of 0.5 foot, (4.6mw/cm^2).
So my backpack radio is within the Texas formula.
Make sure you warn nearby observers that the transmitter might upset Pace Makers, and other medical electronic devices.
Radiation from the Sun is worse than the radiation from the radio, Remember to protect yourself from the Sun.

3. When responding to this article please stay on topic and take all side conversations off of the eHam comments web site.

4. Anyone who is interested in HF Back Pack operation will find a lot of friends on the http://www.HFPack.com web site

5. See my DXCC article at
http://www.eham.net/articles/10734

6. You can see a picture of the Backstop Antenna by clicking on the link at the top of the article:
"Paul Signorelli (W0RW)"
There are more pictures at
http://photos/groups.yahoo.com/group/milpack/lst
then go to the "W0RW Ped" Folder
These Backstop Antenna photos were taken by Mark, KI0PF

7. Please put your real name and your email address in your posts.
Several people who asked questions have no eHam profile or QRZ addresses and i could not contact them.

Paul
w0rw at aol.com
 
RE: Rare Pedestrian Moblile Contact with UK  
by WA1RNE on August 9, 2005 Mail this to a friend!
From WA5ZNU:

How about this calculation?

With a 5W 2 Meter HT on FM at 50% talk time, the same Texas calculator shows you need to keep it 10 inches away and not come closer than about 2 feet to anyone else. That's assuming your rubber duck is a 0dBi radiator:


Actually, your assessment for a 5W 2 FM station is not really correct. The duty cycle is 100%, not 50%- so the MPE of 1.71 is for 5 watts @ 0dBi, not 2.5.

At 2.5 watts under the same conditions, the MPE drops to 0.86 which is within the exposure limit of 1.00.


This example is not exactly apples for apples, with the hand-held & rubber duck probably worse than the backpack scenario.

Considering the gain of a rubber duck is likely to be less than 2.2dBi, say 0-2.2 dbi, it's tuned against the case of the handheld and/or the operator so the power transfer is likely to be lower than expected, the frequency is 8 times higher - 18 mc vs. 145 mc, etc.

Most rubber ducks on 2 meters are essentially linear loaded 1/4 wave verticals that are influenced by close proximity of the operator and other objects, it stands to reason that they are inefficient radiators and are very likely to be less efficient than say a properly designed 1/4 wave vertical sitting in the clear.

Bottom line, the mix of low antenna efficiency and power transfer but at a higher frequency probably negates each other.

A safer alternative might be a halfwave vertical, like the AEA HotRod which would shift the current maximum above the operator's head - instead of the person positioning themselves right at the feedpoint.


I guess the other big factor is really how much operating time you really spend.


But as Vito pointed out, some like standing in front of Rhombics at 20 paces......but not me, thanks.


73, Chris

 
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