[OZAPRS] Newb HF questions ;)

Tue May 19 22:38:14 EST 2009

Hey Norm

I'm a little confused by the balun too ;)

50 ohms u/b from the rig to the atu
atu is u/b (50 ohm?) to the PL259 on the bottom of the balun.
balun balanced side has one terminal to the bottom of the TEV-1, and  
one side to the 'radials' terminal on the mounting plate for the  
TEV-1, which is bolted directly to the mast.  I can see why you'd have  
the 4:1 balun if you were feeding a dipole, but as ray said earlier,  
on 30m, the TEV-1 is pretty damned close to 50 ohms, so I would have  
thought putting the balun in there would completely screw the vswr??   
(it'd be giving me ~11 ohm on 30m yeah?)

Pic of balun installed before I installed the radials (they come off  
the lower U-bolt) - http://photos-e.ak.fbcdn.net/photos-ak-sf2p/v646/250/124/506862762/n506862762_1735428_1347613.jpg

Cheers,

Damien

On 19/05/2009, at 9:48 PM, Norm wrote:

> Damien, Ray et all,
> I'm a little confused by the balun bit. Output of the rig is 50ohms  
> u/b?
> Input to the atu is  50ohms u/b? output of he atu is ??ohms but
> unbalanced as is the input to the load. Where does the "bal" part of  
> the
> balun point?
>
> It's possible that you have a very high vswr on the coax between the  
> atu
> and antenna. That would make the outer of the coax "hot" resulting  
> in RF
> in the shack big time!!!
>
>
>
> Norm, VK3XCI
>
>
> On Tue, 2009-05-19 at 18:27 +1000, Damien Gardner Jnr wrote:
>> Hi Ray,
>>
>> *grin* I thought I'd include as much info as possible ;)  All my RF
>> work has always been on the upside of 100mhz, so I'm totally not sure
>> what's actually to be expected down low :)
>>
>> Re band, at this stage, I'd be happy to be able to work *any* HF
>> aprs ;)  The ultimate goal is to have the rig in the car whenever we
>> go away, but want to see it working in the shed first..  And with the
>> hell it's causing the computer/monitors/etc so far, there's no way  
>> I'm
>> puttiing it *anywhere* near my commode!
>>
>> I don't really get a choice of having the AT anywhere but with the
>> radio - the control cables from the radio to the AT are only 10cm
>> long ;)  Plus, while very rugged, it doesn't exactly look waterproof.
>> I have to wonder whether I actually should have the 4:1 balun on  
>> there
>> though - I wouldn't have thought so with it being a vertical, but  
>> this
>> is my first foray into HF, and the bloke at Andrews swore blind I'd
>> have to have it in there...  Re the radials, I wasn't sure that  
>> they'd
>> really have any use, since the antenna is only mounted 2.4m off the
>> ground, but again, the bloke at Andrews swore they had to be there..
>> That and the mounting to the shed isn't the strongest, and with the
>> wind we get here, I thought they'd work nicely as a guys too ;)
>> Reading up in my RSGB, it seems that that close to the ground,  
>> they're
>> not really that much use, though it did suggest that they definitely
>> shouldn't be resonant..  One's ~6m, one's ~5m, and the third is ~7m.
>> Their grounding at the far end is basically clamped onto a tent peg -
>> I'm wondering given that they don't do much, whether there's any  
>> point
>> replacing them with 1m long stakes? Probably not?
>>
>> Re the shielding stopping at the ATU, are you saying that the shield
>> on the cable coming out of the ATU to the antenna is no longer at
>> ground, or just that it's at ground *at* the ATU connection, but
>> because of the impedance mismatch at the antenna, it gets further  
>> from
>> ground as it gets closer to the antenna?  That would certainly  
>> explain
>> so much RF in the shack - only the last 2 feet are outside the shack!
>>
>> Given that I basically only want to work one band once I'm sorted,
>> will this RF-in-the-shack problem go away if I just use a single-band
>> mobile antenna on the roof of the shack?  I went the TEV-1 with an
>> auto-tuner so I could play around and work any band, but if using a
>> fixed-band mobile antenna will make things 'work', that's probably
>> going to be muuuch easier..
>>
>> And lastly, re the antenna being right above the shack causing the RF
>> inside, the shack being totally metal, and earthed wouldn't stop  
>> that?
>>
>> Cheers,
>>
>> Damien
>>
>> Damien Gardner Jnr
>> VK2TDG. Dip EE. GradIEAust
>> rendrag at rendrag.net -  http://www.rendrag.net/
>>
>> On 19/05/2009, at 5:44 PM, Ray Wells wrote:
>>
>>> Hi Damien,
>>>
>>> Crikey! I had to wade through lots of QRM to get to the guts of your
>>> query  ;-)
>>> My first response is to cite an old RF engineer from many years ago
>>> who advised to get the RF into the antenna and not the shack by
>>> using a resonant dipole and always use a balun. However, that isn't
>>> always practical, and you have a vertical.
>>>
>>> Given that you are referencing HF and aprs, can I assume you mean
>>> 30m? You didn't say.
>>>
>>> Let's assume you're using 30m.
>>>
>>> An 8.5 metre vertical on 30m is about 0.287 of a wavelength, just a
>>> bit over a quarter wavelength, making its impedance around the 40
>>> ohm mark, with some inductive reactance. A series capacitor (use a
>>> variable) would quickly enable you to tune the antenna to resonance,
>>> and achieve an excellent match to 50 ohm coax. A nice low impedance
>>> feed point to minimise radiation from things like the feedline (you
>>> did say you had radials - are they resonant?)
>>>
>>> If you are using 40m for aprs your antenna is going to have a fairly
>>> low impedance, with a high capacitive reactance, requiring series
>>> inductance to achieve resonance - but that alone won't fix the
>>> impedance mismatch. Also, the radials are likely to be largely
>>> ineffective, unless they are resonant. However, given the low
>>> feedpoint impedance that will result in higher feedpoint currents, a
>>> simple array of three radials will be close to useless, albeit
>>> better than nothing.
>>>
>>> You mention the ATU is bolted to the radio. That means the shielding
>>> provided by the coax from the rig finishes at the ATU. If you are
>>> running coax from the ATU to an antenna that is not resonant, forget
>>> about the coax providing any advantage, you WILL have RF in the
>>> shack. There is ONLY one place for the ATU and that's at the  
>>> antenna.
>>>
>>> Of course, if the shack is close to the antenna, your shack
>>> equipment will likely be affected by radiation from the antenna no
>>> matter how well the antenna is fed and matched. My HF antennas are
>>> single-band, balun fed dipoles up 45', and 150 feet from the shack,
>>> with the coaxial cables - most FHJ4-50 - buried in ducts. I still
>>> see occasional RF problems, and the CRO goes berserk.
>>>
>>> Ray vk2tv
>>>
>>> Damien Gardner Jnr wrote:
>>>> Howdy Folks,
>>>>
>>>> So I've *finally* unpacked all my gear from their boxes, fixed a
>>>> few faults (seems a dry joint on the output DIN socket was why the
>>>> old MFJ running the vk1 igate went silent), and thought I was ready
>>>> to play APRS on HF..  But it seems not..  After doing a lot of
>>>> playing about with the help of an immensely more RF-knowledgable
>>>> colleague, I've come to realise that I live in a pretty horrid area
>>>> to be trying to do anything RF-related in, but I'm going to try and
>>>> struggle through that..
>>>>
>>>> Backstory - when we shifted here from 10km down the road, we had to
>>>> replace our 900mhz cordless phone with a 5.8ghz DECT setup, as the
>>>> mains hum on the old one was pretty horrid..  When we got a baby
>>>> monitor earlier this year (yep, I added to the pool of hams with
>>>> girl children.. seriously, does ANYONE in our hobby have boys?!?),
>>>> we had to replace that with a DECT unit, as that was absolutely
>>>> horrid..  and then that DECT unit had to be replaced with one from
>>>> a different manufacturer, as even IT had 50hz mains hum..  I
>>>> actually spoke to the bloke that designed the DECT one we had
>>>> problems with, and he was pretty concerned, as he figured the only
>>>> place it'd be getting in was actual mains proximity to the
>>>> receiver.. - but it was doing it in the carpark at the local
>>>> shopping centre!  But stopped 10km down the road..  APRS on my D7
>>>> is completely deaf, even though RAG hears me fine, and my FT8100R
>>>> with a 9db colinear on the roof of the shed can hear it and RAG
>>>> fine - but get more than a block or two away with the D7, and the
>>>> 8100 no longer hears it even at 5W output!?
>>>>
>>>> Anyway, back to my problems, and questions. If there's a list more
>>>> appropriate, PLEASE point me at it ;)  This is just the only ham-
>>>> related list I'm on (apart from the APRS admin list..)
>>>>
>>>> So I'm finally setup - the FT8100R with a 2/70 9/6db mobile antenna
>>>> (used to be on my luxie) mounted above the roof of the shed (http://photos-a.ak.fbcdn.net/photos-ak-sf2p/v646/250/124/506862762/n506862762_1735424_1072471.jpg
>>>> ).  And an FT-897 with an LDG AT-897 feeding RG-58U (couldn't see
>>>> any point running LMR-400 for an 8-meter run on HF..?) to a TEV-1
>>>> (8.5m aluminium vertical ) via a 4:1 balun (http://photos-f.ak.fbcdn.net/photos-ak-sf2p/v646/250/124/506862762/n506862762_1735429_1155875.jpg
>>>> ). (There's been three radials added since that shot - 120 degrees
>>>> apart, at around 40 degrees down, grounded at the ends, and bonded
>>>> to the 'radials' point on the antenna end)
>>>> The pole the TEV-1 is mounted on is rammed about 20cm into the
>>>> ground at the base, and is touching the shed roof about 50cm below
>>>> the base of the antenna - not sure on whether I should be
>>>> insulating that point or earthing it, or leaving it as is?
>>>>
>>>> The shed is thin steel sheet, and is earthed both to the AC earth,
>>>> and via a 1m steel rod driven 90cm into the ground (seriously, have
>>>> you seen the cost of copper clad earth rods recently?!?), with 5mm
>>>> steel cable (I have *so* much of the clear plastic-covered 5mm
>>>> steel cable left over from old microwave link installs, and it fits
>>>> perfectly into 5mm crimp connectors ;) ) to the frame of the shed.
>>>> There's also a run of cable bolted from the frame of the shed to
>>>> the -ve terminal of the 60A 12V supply, and to the gnd connection
>>>> on the back of the FT-897.
>>>>
>>>> So anyway, apart from having S8-9 noise below 18Mhz, which I'm just
>>>> going to try to work around, I'm fairly sure there's something not
>>>> right in my setup.. - Any time I hit TX below 20Mz, my USB mouse
>>>> turns itself off (has to unplugged and re-plugged), my two LCD
>>>> monitors go wavey (like old crt's would when you hit tx with a
>>>> handheld near them..), and all the serial ports into the pc in the
>>>> shed start having random crap appear on them..  Since I'm seeing
>>>> the meter movements in my SWR meters (neither of which are plugged
>>>> into anything at the moment, though are powered up..) are coming up
>>>> to about 1/2 a watt, I got to and pluged a probe into the cro, and
>>>> attached a metre of wire to the tip.. -  I'm seeing 1V p-p of
>>>> signal (tx'ing on am, a nice clean sinewave at the tx frequency)
>>>> inside the shed, which I wouldn't normally expect, especially with
>>>> the shed frame (and it's a completely steel shed..) being grounded?
>>>> I would have thought that with the case of the 897 grounded, an the
>>>> antenna tuner bolted to the 897, there shouldn't be any radio
>>>> signal anywhere but the coax, and that shouldn't be radiating??
>>>> Plugging the 2m/70cm antenna's cable straight into the cro, I see
>>>> 5v P-P, but that's outside the shed, so I'd kinda expect that..
>>>>
>>>> SO.. After that extremely long blurb, does anyone have any
>>>> thoughts? *should* I be seeing that much RF inside the shed?  If
>>>> not, where should I look next?  Is it worth reaming a hole in the
>>>> shed wall, and make up a run of LMR-400?  I wouldn't have thought
>>>> that the RG58 would be an issue, but maybe?  Or should the majority
>>>> of the coax run be outside the shed, rather than inside? (again
>>>> though, I wouldn't have thought the coax would be radiating?)
>>>>
>>>> Thanks!
>>>>
>>>> Damien
>>>>
>>>> Damien Gardner Jnr
>>>> VK2TDG. Dip EE. GradIEAust
>>>> rendrag at rendrag.net -  http://www.rendrag.net/
>>>> -- 
>>>> We rode on the winds of the rising storm,
>>>> We ran to the sounds of thunder.
>>>> We danced among the lightning bolts,
>>>> and tore the world asunder
>>>>
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