Sunday, February 23, 2014

Receive antenna experiments at ED5T

This weekend is the CQWW 160 phone contest. With one thing and another, we decided not to actively operate in the contest. However, Elías EB5KT and myself arranged to go up on Saturday night and take advantage of the fact that there was a contest on, in order to do some RX antenna tests.

ED5T is at a great location for the high bands. Our stacked yagis for 10 and 15 metres do great, as does the tribander for 20. We are at the top of a ridge with a nice drop off to most areas of ham population. However, on the low bands, we do not have enough height for good horizontal antennas, the ground is poor for vertical antennas (mostly bedrock), and it is extremely noisy on reception.

For transmitting, we now have an inverted L for 160m, which appears to work, but we don't really know since we have a very high noise level and can't really tell who is calling us. In the past, we have tried the K9AY design, long terminated beverages, and the flag. None of these antennas gave particularly good results. The K9AY was forcibly located near to a metal fence which we think probably degraded performance. The beverages perhaps didn't work too well since local terrain dictates they can't go perfectly straight, nor at a fixed height from ground all along the antenna. Also, these antennas need to be installed before each contest, and for the beverages in particular, hauling two 500m long antennas through the forest and then attaching them to trees is not a load of fun. Whilst both designs work, they do not provide a significant improvement over the transmitting antenna and therefore we continue on our quest for a design more suited to our needs.

One of the tests Elias wanted to do was to use a portable receive antenna to try to find out if there are any locations close by which are better than others, either in noise level, signal strength, or both. In order to do this, he made a monoband Shielded Loop antenna for 160 on a portable pole. We used an FT857 radio with a small battery in a backpack and started testing in different locations.

The results were very interesting:

- We stopped in around 15 different places and tested for noise reduction by turning the loop.

- In most places we stopped, by turning the loop we were able to obtain a null in the noise and except when close to buildings or right under power lines, we could usually reduce from between S7 and S9 peaks down to S0 noise

- We could not pointpoint a particular direction for the noise. In some cases, only moving 10 metres produced a 90º difference in the null location. We are unable to account for this.

- In the case when listening to a frequency occupied by a station calling CQ, in most cases the noise null did not coincide with a reduction in signal strength of the station calling (máximum signal to noise ratio).

- When listening to ED5M, we were able to hear about half the stations they were working. They were using a full size 2 element delta loop at 50m (150') high for TX, plus numerous 650m long beverages and a K9AY, in an electrically quiet natural park about 100km from us.

- We sat the antenna on the top of my car and plugged into my TS590. The absence of noise made the fatigue factor usually present, neglible. It was almost like working on 10 metres. We were able to hear a large number of contest stations without any problem.

The results are not conclusive since we did not use any know reference antenna for comparison (rule nº1 of basic science). However our experience dictates that this was a positive experiment and worth pursuing. For next weekends ARRL contest, Elías hopes to have a Pennant antenna built and install it at one of the locations we found to be good, and that is reasonably close to the shack.

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