Topband: INV-L with traps

[Alex]

Hi Cristi,

I built the BCS antenna about 3 weeks ago and I used RG-59 with solid
dielectric for the traps. I build the traps to resonate in band (using the
MFJ-269 as a grid dipper). I wish I had read Tom's, W8JI, article on traps
before I built them, then I would have the traps resonate out of band to
lower losses. 

I usually do not worry too much about the SWR of an antenna system, as long
as the feed line losses are low. I used only 22 radials varying in length of
30 to 135 feet. I also have a ground screen just below the feed point from
chicken wire. It's small, probably too small to matter. In any case, I got
the following measurements at the feed point:

40m: resonance at 7120kHz SWR 1.1 Rs=40 Xs=0
80m: resonance at 3.832kHz SWR 1.2 Rs=37 Xs=0
160m: resonance at 1813kHz SWR 2.5 R=20 Xs=9 (best SWR at 1852kHz SWR 1.2,
RS= 50 Xs=14)

The radial system is not really sufficient for 160m, which is to be expected
with only 22 radials.

The vertical portion is about 55 feet and the rest slopes down at an angle
of 45 degrees. Not ideal, but the best I can do in my tiny back yard.

The length of the 40 section is 31ft, 15 feet for the 80m section and 41
feet for the remaining section of 160m.

On 40m I found that the antenna was doing about 6dB worse than the other
antennas I have for this band, which is a 40m doublet at 52 feet towards EU,
and an 80m inverted V at 55 feet. That made me take out the 40m trap and
retune the antenna so that it's now just a dual band antenna. 

I'm not surprised the antenna wasn't doing well on 40m because I have houses
and trees all around me. So having an antenna with the maximum current quite
close to the ground where there are a lot of obstructions is just not going
to work well.

On 80 meters the story is different. The inverted-L works as well or a
little better as the 80m inverted V at 55ft on DX. Close in signals are
usually a little stronger on the Inverted V. I'm not going to quantify that
because the differences are too small to measure on a probably inaccurate
s-meter of the ProIII. 

I have nothing to compare the antenna with on 160m. I don't think it's doing
any worse that the coaxial inverted L I had in its place before.

Still, it's too early to pass judgment on this antenna in my situation. I
need more experience with it.

Anyway, I thought I'd share with you what I got so far.

73,
--Alex KR1ST
http://www.kr1st.com
http://www.airlinkexpress.org


> -----Original Message-----
> From: topband-bounces at contesting.com [mailto:topband-
> bounces at contesting.com] On Behalf Of cris blak
> Sent: Thursday, October 08, 2009 1:51 PM
> To: topband at contesting.com
> Subject: Re: Topband: INV-L with traps
> 
> Hello all,
> 
> Last weekend I succeed to install the two traps on my 160m INV-L to be
> used on 40 and 80m. The traps are made by RG58 coaxial cable wind up on
> 40mm plastic pipe. The 40m has 11 turns and the 80m one, 21 turns. Each
> of theme were checked on the bench for resonance and it did resonate on
> those bands.