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[email protected] · May 3rd 12, 06:24 PM posted to rec.radio.amateur.antenna

wrote:
On 3 May,
wrote:

wrote:
On 3 May,
wrote:

I was refering to the case where there is nothing to adjust the match
and getting something under 3:1 at the end of the feed line.

It was well within 1.5:1 within (part of) 10 and 12m bands.

After spending several hours with EZNEC trying to get that combination
under 5:1 with no success, I find that hard to believe unless something in
the system is really lossy (like the feedline) and masking the SWR or the
length of the feedline just happens to be at a length to help the match.

Well I modelled it and matched it fine on the two bands The ends of the
feeder are about 4" apart and effectively the inner ends of the longer dipole
are an 8" length of 600 ohm line, so that length comes off the 12m dipole
elements. I couldn't get it to match with it as a genuine fan (inner ends
paralelled) with any small spacing at the ends of the dipoles, so I made them
about 8" apart.

Then they matched up nicely. The feeder is short, and low loss, as seen by
the out of band (mis)match.

The trick is to make the dipoles far enough seperated so as to be seen by the
feeder as seperate elements. This lost me much time.

This comes after my earlier reply about wanting to see the model.

When I did the modeling mentioned above, I modeled the dipoles in a fan
configuration connected to a common short connection and varied the angle
between the dipoles and the length of one dipole.

I just did another series of models this time as parallel dipoles separated
by 6 inches connected to a short (1 inch) common section for the feed
point.

The top dipole was set to be roughly resonant at 30 Mhz and the lower
dipole was initially set to be twice as long.

After saving the SWR plot, I repeated for length ratios of 1.5:1, 1.25:1,
1.1:1, and 1.05:1.

Short summary:

The top dipole remained fixed in length but the lowest SWR point shifted
slightly for the higher frequency.

The lowest SWR's at both frequencies were both under 1.5:1 over the range
of 2:1 to 1.05:1 for the length ratios.

Therefor either fan dipoles behave differently than parallel dipoles or I
screwed something up in the previous run.

I'm going to have to run fans again and this time save the results.

If anything interesting turns up, I can put all the data on a web server
somewhere and post it for those interested.