[OZAPRS] Newb HF questions ;)

Ray Wells vk2tv at exemail.com.au
Tue May 19 17:44:39 EST 2009


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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> Ozaprs at aprs.net.au
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