Skip to main content

20m vertical

Antennas are always an issue with neighbours, family, town planners etc. So to date I have limited them to either a) discrete long wire or b) 'TV' like antennas e.g. 2 el band 1 TV antenna for 6metres. However, I had a need for a decent 20m DX antenna and the obvious choice was a vertical. The ideal design was for a full 1/4 wave vertical on the gable end with two 1/4 wave radials sloping down the gable ends. So with the following raw materials:

1) glass fibre fishing pole (7m)
2) 3 x 17ft lengths of 2mm wire
3) Self amalgamating tape
4) 10m UR67
5) 'N' type bulkhead connector
6) chimney brackets and u bolts
7) 2 x dogbone insulators and 2 x standoffs

The vertical wire was run up the inside of the fishing pole and soldered to the N type bulkhead. The two radials were then soldered to the N type and the fishing pole shoved over the vertical element. Self amalgamating tape then sealed the bottom and top of the fishing pole. Initially the top section was left off to enable trimming of the length. Surprisingly, when the final pole section was placed over the top of the antenna the vswr shot up to 3: 1. This was probably due to some strengthening wire in the final section and consequently this section was omitted. Note the SOTA poles are probably ok in that they are glass fibre throughout their length. The antenna was mounted on gable end of house as shown in photos. The 17' vertical and radials were trimmed for best SWR , and a number of different radial angle (to horizontal) were tried. The final best SWR 1.3 : 1 (in 50 ohms) was obtained with the radials actually on the gable ends - which looked best as well. The gable ends being less than 17' required the ends of the radials to be bent down and attached to the drain pipes (insulated from them). With 70 watts CW, the log for the first 2 days included:


RK4HZ Samara 0730hrs
JJ1KXB Tokyo 1200hrs
UT5KTT Zlolbunov 1315hrs
UU9CI Sevastopol 1700hrs
VE2DOH Montreal 2020hrs

RW0BM Dudinka 0810hrs
K1GCD Maine 1938 hrs(UTC)
K2TQC Syracuse 1830hrs
LU2FLN Santa Fe 2100hrs

Note also that the standard length calculation for a 20m 1/4 wave element of 2mm thickness is 16ft 11.5 in at 14.050 MHz.

URM67 Specifications:
Overall diameter: 10.3mm
Capacitance (core to screen): 100pF/m
Max. voltage: 6.5kV
Nominal impedance: 50Ω
Attenuation per 10m: 0.68dB at 100MHz and 2.52dB at 1000MHz.

Say approx 0.2dB at 14Mhz for the 10m run to the antenna. This is a power ratio of 1.047 which equates to a loss of around 3 watts in the 70 from the Tx output. I can just about live with this and the affect on Rx noise figure is a straight 0.2dB which is negligible.
Labels:

Comments

Post a Comment

Popular posts from this blog

Digital Bandpass Filter FIR design - Python

The python code generates the Finite Impulse Response (FIR) filter coefficients for a lowpass filter (LPF) at 10 (Hz) cut off using firwin from scipy.  A highpass filter is then created by subtracting the lowpass filter output(s) from the output of an allpass filter. To do this the coefficients of the LPF are multiplied by -1 and 1 added to the centre tap (to create the allpass filter with subtraction). A second LPF is then created with a cutoff at 15 (Hz) and the bandpass filter formed by addition of the LPF and HPF coefficients. The program also generates a test sine wave of a given amplitude and power and to this noise from a Normal distribution is added.  The graph below shows the signal and nois, and the signal (green) after filtering. The input snr is approximately 3dB. The frequency response below shows the passband centered on 12.5 (Hz), the Nyquist frequency is 50 (Hz). from numpy import cos, sin, pi, absolute, arange from numpy.random import normal fr...
Post a Comment
Read more

FT 817 Power Amplifier

This very simple 2 Fet power amplifier easily achieves 250W output with an FT817 5W drive. The key design details as follows: 3:1 broadband input transformer matches the 5.5 ohm gate load resistor (4 x 22 ohms in parallel) to the 50 ohms required by the FT817 . The 4:1 output broadband transformer presents 3 ohms (16:1 impedance ratio) to the balanced HEXFET pair each mounted on a 3mm copper heat spreader which is insulated from the 2 1w/degC heatsinks. These are blown cool by a fan underneath. The power supply required is 28v at 30 amps. The amp is around 50% efficient with a standing 750mAmp temperature compensated bias. An IC 703, with 10watts output will drive the output to around 400 watts. The output filter shown is a 5 pole topband filter with T130-2 torroids and 400v silver mica caps. Peak output voltage on 160 metres with 5 watts drive is 160v or 320v p-p in 50 ohms equating to 250watts. This is slightly higher than the reading on the 3kw MFJ power meter. The inline F...
4 comments
Read more

Splunk Cheat Sheet (Linux)

1. set root's password:  sudo su passwd root Enter new UNIX password: < new_root_password > Retype new UNIX password: < new_root_password > passwd: password updated successfully # su - 2. Remove any existing Splunk directories & create user etc: # rm -rf /opt/splunkforwarder # userdel -r splunk # this will remove as above if user splunk's home directory # groupadd siem # useradd -g siem -s /bin/bash -d /home/siem -m siem # vi ~/.profile # chage -I -1 -m -0 -M -99999 -E -1 siem If above fails because of multiple passwd fails: # pam_tally --reset check with #chage -l siem # uname -a # check OS version # dpkg -i splunk-4.3.1...........intel.deb # chown -R siem:siem /opt/splunk # su - siem : $SPLUNK_HOME/bin/splunk start --accept-license : $SPLUNK_HOME/bin/splunk edit user admin -password newpassword -role admin -auth admin:changeme 3. vi ~/.profile (as follows) (OR .bash_profile) # ~/.profile: executed by the command interpreter for log...
1 comment
Read more