[AMRadio] bad antenna luck


Jim Candela jcandela at prodigy.net
Tue Jan 9 08:54:56 EST 2007


David,

   One more thing I forgot to mention. A SWR minimum when sweeping frequency across the band means the load is resistive at that point for the fundamental frequency you are transmitting. The SWR can be 2:1. 1:1. or 10:1. When the load is resistive, then the SWR reflects the load impedance referenced to 50 ohms. A 2:1 SWR might mean the load is 25 or 100 ohms. Having an impedance bridge makes this simpler to determine. Knowing the transmitter tuning position for 50 ohms (dummy load) might help determine which way your antenna impedance goes ratio'd to 50. With a pi-network in your transmitter, more loading control capacitance means lower impedance, less loading control capacitance means higher impedance (and more harmonic output).

   I once used a Yaesu FT-757 on AM. Band by band I blew the 'half-octive'  filters between the RF power amp, and the output. I decided to replace that POS board, relays, filters  and all with a piece of wire. As a result the transmitter put out a 3rd harmonic that was strong. When loading into a resonant 1 band antenna, or into a tuner, my SWR minimum was about 1.7:1. This was because the antenna or tuner was reflecting the harmonic energy back to the transmitter even though the antenna or tuner looked like 50 ohms resistive at the desired transmit frequency.

Jim
JKO

----- Original Message ----
From: Jim candela <jcandela at prodigy.net>
To: "Amradio at Mailman.Qth.Net" <amradio at mailman.qth.net>
Sent: Tuesday, January 9, 2007 6:39:27 AM
Subject: FW: [AMRadio] bad antenna luck

David,

   There are days when I cannot make a resistor work, or a light bulb light.
Sounds like you are having a bad antenna day. In times like this I usually
punt and go back to basics. Some ideas:

Use your transmitter, swr meter and a known 50 ohm dummy load. Do you see
1:1 SWR? Answer should be YES. If not find out why.

Use your transmitter, swr meter, coax to coax transmatch, and a known 50 ohm
dummy load. Tune transmatch for minimum SWR. Do you see 1:1 SWR? If not,
then either your transmatch cannot match to 50 ohms load, our your
transmitter is generating considerable 2nd or 3rd harmonics when the
transmatch is tuned to the fundamental frequency only. There have been many
times where I've had a transmitter putting out considerable harmonic energy,
or a spur, where I could not achieve a 1:1 SWR. It wasn't the tuner, or the
antenna. It was my transmitter, and how I had it tuned (loading capacitor
too far un-meshed).

How is your station grounded?

How is your tower grounded?

What is the length of your Feedline?

What type feedline do you use?

Is it one piece?

What band are we talking about? 75, 40, 20m, etc..

What is the angle of your inverted V?

Regards,
Jim
JKO

-----Original Message-----
From: amradio-bounces at w5ami.net [mailto:amradio-bounces at w5ami.net]On Behalf
Of David Adams
Sent: Monday, January 08, 2007 11:54 AM
To: Amradio at w5ami.net
Subject: Re: [AMRadio] bad antenna luck


hi gang i need a little help no matter what i do i cant seem to get a
antenna flat at any freq. on 75 i built a bazooka it worked well for a
little while until it failed but it was never flat not even close i cant
even make it flat with a tuner what AM i doing wrong, looking for
suggestions i can only use inverted vee type ant. the inverted vee i was
using at first worked okay then i moved it from the tree to my tower the
center is up at 65ft. feed point the ends are aprox 15ft. off the ground it
will not tune any lower than 2.5.1 swr even with tuner something i AM over
looking, HELP tnx dave
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