[Lowfer] A couple of crude Antenna Matching techniques - will either one work

Fri Dec 1 11:32:59 EST 2017

Hi Andy,

The way that I did it back in my big LowFER days (the 80s) and the way 
I've been doing it for both 630 and 2200 meters now is like this:

- Connect the TX to a dummy load and transmit, noting the PA current - 
or the transmit current overall.

- Connect the TX to the matching network.  Using the RF ammeter (on the 
"antenna" side of the matching network) maximize the antenna current.

- Adjust the impedance to get the same PA current as was noted when 
using the dummy load.  In my case my 50 ohms TX PA output goes to an 
autotransformer (about 25-30 turns on an FT240-61 core, I think - about 
200uH total, tapped every 3 turns from the "hot" end down to about 20% 
from the "ground" end.)

For 630 meters, I tap way down on the autotransformer - probably in the 
15 ohm area based on the number of turns at the "output" tap (to the coil).

As far as getting an HF meter to work, mine reads "low", but the FWD/REF 
ratio seems "believable" - but it is a bit off, probably due to some 
sort of compensation that I'd need to do.

* * *

One device that is frequency-agnostic is the so-called "Tayloe Bridge" 
which is just a resistance bridge that substitutes the TX and ANT for 
some of the legs.   It is built with resistors that set the expected 
load impedance, so it could be made for practically any Z.

Being just resistors (with a diode and a meter and/or LED in the middle) 
it doesn't care about the frequency and it has the advantage that the 
VSWR through it (when it is in-circuit) can never by very high as it 
dissipates much of the TX power.  It's not something that you'd ever 
want to leave in-circuit, but it's simple and easy to build and is 
practically foolproof.  Without additional sensing it can't directly 
read VSWR so it is only useful for matching (e.g. nulling the detection 
LED/meter) but that is often all that one needs.

* * *

FWIW, my TX antenna is my HF "lazy loop" - a loop that is horizontal, 
about 300-ish feet in circumference and between 25 and 40 feet up fed 
with 450 ohm window line.  I simply connect both ends of the window line 
together and feed it via a homebrew variometer (150-225uH) and "push" 
against various grounds - notably the station ground which consists of 
several ground rods and a couple of abandoned, buried CATV cables as 
well as my "house" ground which also includes the metal roofs of my 
house and (separate) garage.

For TX I use a homebrew (5 MHz IF = 10 MHz OCXO/2) transmit converter 
driven by an "opened up" FT-817 driving a single-ended FET PA.

With 20-30 watts of RF I'm able to manage about 2 amps of RF on 630 
meters and about 900mA on 2200 (I can draw a 3/4" arc from the output of 
the loading coil on either band!) and I typically get equal or better 
signal reports on JT9 than the station I'm receiving in QSO on 630 
meters, being able to work anyone that I "hear" so far and even managed 
a smattering of CW contacts.

On 2200, I've only run WSPR, being heard by 3 stations over the past two 
nights, but there are relatively few monitoring stations on that band at 
the moment.

73,
Clint
KA7OEI

On 11/30/2017 11:02 PM, Andy - KU4XR via Lowfer wrote:
> Greetings all:
>
> When I reach the point that I want to put some RF into my antenna on 630 meters, I have a problem...
> I have no accurate way to know that the matching is correct - bringing the  antenna to resonance.
> I have no equipment on hand other than a SWR meter, and a couple  of RF Ammeters..
> I have 2 scenarios - are either of these able to make the matching close enough that I will not be burning up finals ?
> #1 : The SWR meter will not be accurate, but it " should " at the least show a null .. If I put a RF Ammeter at the
> top of the loading coil, and the SWR meter at the bottom of the loading coil, If I can find the Tap / Cap settings
> that show maximum RF current on the antenna, and the SWR meter is at null .. Will the
> match be close ? #2 : this same method was mentioned to me by using 2 - RF Ammeters.
> One on each end of the coil... when I find the Tap / Cap settings that will show the same RF Current on both meters;
> then the antenna should be very close to resonance .. the variables in this method would be preferably having
> 2  Identical RF Ammeters, and hope they are in correct calibration... All opinions, and experience with this will be
> most appreciated..
>
> Thanks, and 73 to all:
>
> Andy - KU4XR
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