Posts Tagged ‘70cm’
6m/70cm antenna updates at the QTH
April 18th, 2016 | 
Author: 
The 70cm yagi was to replace the small 7-element one which I pressed into service as a hand held antenna for potentially finding my HAB payloads last year.
To be honest I wasn't really after a 6m antenna due to the size and visual impact. I have a 6m 'wooden' moxon I made back in 2014 but it is heavy, ugly and unstable so had been collecting cobwebs in the garage. I decided with the prospect of 'Sporadic E' season around the corner I would be foolish not to put it up.
The HB9CV wasn't in the best of conditions but seemed complete albeit it was purchased dismantled. The elements were a little weathered so I set about cleaning them up with a light rubbing down with some wire wool and a rag with a drop of WD40. On closer inspection the tube that made up the front element had some noticeable bending and on removing the plastic mounting to investigate I spotted a crack in the tube wall by the hole for the mounting bolt.
While it hadn't totally separated it would only be a matter of time before it did fail as it was flexing, the plastic mount was the only thing holding it together. I found a suitable piece of alloy tube from a scrap antenna which was a perfect fit inside the broken element. I cut a suitable length and pushed it up inside the element to the appropriate position and then simply drilled through and bolted either side of the central hole to stabilise and strengthen it (can be seen in image below)
The next issue I had to address was the feed point, it came with about six inches of RG58 coax projecting from it where it had simply been cut for removal by the previous owner. I prised off the cover cap to find it full of water, the reason being a hole in the back and it being stored outside I believe.
Thankfully the trimming capacitor seemed okay, while it was wet it wasn't corroded. It was all dried out and the hole plugged with silicon and I set about re-assembling the antenna which was a little fiddly to get the phasing line to sit properly but once done it was a simple matter of adjusting the capacitor to get the VSWR to a minimum in the SSB section of the 6m band. I mounted on the rotator pole just below the X50 collinear.
I used it last month in the 6m UKAC and while my operating not exactly earth shattering I was happy with its performance using just 10W in the low power section just "search and pouncing" for a little over an hour.
.
The 70cm Yagi was straight forward as was already assembled, I just had to make a slight tweak to the gamma-matching bar as the VSWR was unexpectedly high around 2:1 in the SSB segment of the band, it seemed to have been tuned for the FM portion of the band. I managed just an hour "search and pouncing" in Aprils 70cm UKAC, I started late and while signal reports both ways were a marked improvement I found the extra directionality and off beam rejection something I will have to get used not helped by a temperamental rotator.
6m/70cm antenna updates at the QTH
The 70cm yagi was to replace the small 7-element one which I pressed into service as a hand held antenna for potentially finding my HAB payloads last year.
To be honest I wasn't really after a 6m antenna due to the size and visual impact. I have a 6m 'wooden' moxon I made back in 2014 but it is heavy, ugly and unstable so had been collecting cobwebs in the garage. I decided with the prospect of 'Sporadic E' season around the corner I would be foolish not to put it up.
The HB9CV wasn't in the best of conditions but seemed complete albeit it was purchased dismantled. The elements were a little weathered so I set about cleaning them up with a light rubbing down with some wire wool and a rag with a drop of WD40. On closer inspection the tube that made up the front element had some noticeable bending and on removing the plastic mounting to investigate I spotted a crack in the tube wall by the hole for the mounting bolt.
While it hadn't totally separated it would only be a matter of time before it did fail as it was flexing, the plastic mount was the only thing holding it together. I found a suitable piece of alloy tube from a scrap antenna which was a perfect fit inside the broken element. I cut a suitable length and pushed it up inside the element to the appropriate position and then simply drilled through and bolted either side of the central hole to stabilise and strengthen it (can be seen in image below)
The next issue I had to address was the feed point, it came with about six inches of RG58 coax projecting from it where it had simply been cut for removal by the previous owner. I prised off the cover cap to find it full of water, the reason being a hole in the back and it being stored outside I believe.
Thankfully the trimming capacitor seemed okay, while it was wet it wasn't corroded. It was all dried out and the hole plugged with silicon and I set about re-assembling the antenna which was a little fiddly to get the phasing line to sit properly but once done it was a simple matter of adjusting the capacitor to get the VSWR to a minimum in the SSB section of the 6m band. I mounted on the rotator pole just below the X50 collinear.
I used it last month in the 6m UKAC and while my operating not exactly earth shattering I was happy with its performance using just 10W in the low power section just "search and pouncing" for a little over an hour.
.
The 70cm Yagi was straight forward as was already assembled, I just had to make a slight tweak to the gamma-matching bar as the VSWR was unexpectedly high around 2:1 in the SSB segment of the band, it seemed to have been tuned for the FM portion of the band. I managed just an hour "search and pouncing" in Aprils 70cm UKAC, I started late and while signal reports both ways were a marked improvement I found the extra directionality and off beam rejection something I will have to get used not helped by a temperamental rotator.
2m big-wheel – 3dBd gain without rotating
Since its erection yesterday in its proper home by David G0LRD all seems to be working well.
A quick check this morning and I am able to copy the Kent, Dutch and Belgium beacons on 2m and GB3LEU (NW of Leicester) beacon on 70cm with the FT817 and 2m big-wheel. This is a single 2m big-wheel (not a stack) from Wimo in Germany.
It has 3dBd horizontal gain and is omni-directional. It is an ideal SSB/CW/data antenna for casual operators like me. “Real” operators would use it and a beam: the big-wheel to watch all 360 degrees and a big gainy beam to pounce in the choice direction. Being one who does not take the hobby too seriously just the big-wheel suits me and my style of operating! I did not look for other beacons. It is nice not having to worry about a beam and rotator.
I have still to try this antenna (and feeder) as an HF vertical via my ATU. It should work on at least some HF bands.
Coat hanger 70cm beam
Some years ago, I was in need of a 70cms antenna for portable use and had to improvise by raiding my XYL’s wardrobe! The result was a 2 el Moxon antenna made in about 30 minutes which worked some impressive QRP (2.5W) DX into France on 70cms. Gain is quite low but all you need is a wire coat hanger and a few bits for insulation and to attach the coax feed. Simplicity! It does not look pretty, but it works. The mast was made from a short length of PVC pipe.
See https://sites.google.com/site/g3xbmqrp3/antennas/70cm_moxon .
70cm AM test with low cost modules
This evening I listened for G6ALB on 70cm. Andrew is 3km from me. We both have V2000 vertical co-linear antennas. He was using a very low cost SAW locked TX module, which produced as much FM as AM when modulated with an electret mic. The biggest problem was Andrew’s low power and low level of modulation. He needs more mic gain, more TX power (10dB more?) and some pre-emphasis on the audio. TX power today was very low milliwatts. From 433.925MHz up to at least 433.990MHz a lot of squeaks and whistles could be heard, presumably from more local ISM devices. I was receiving G6ALB using an FT817 at about RS41 on FM and weaker on AM. This very initial test produced results that were expected. To use a super-regen module on RX (the intention is a very simple AM voice transceiver) will require G6ALB to be considerably stronger than on these first tests.
433MHz AM modules – G6ALB progress
G6ALB has reported some progress in his tests using 433MHz AM modules, although it looks like an external linear audio transistor stage may be necessary to get best AM (voice) sensitivity from the super-regen detector. On TX he is getting 10dBm from the TX module at 5V (more with higher supply voltages) although the mod seems to be a mix of AM and FM. Our first “DX” test will probably be from G6ALB to me using the voice modulated TX module. I’ll receive Andrew on my FT817ND initially. When the super-regen RX is sensitive with speech I’ll try to receive him with that. Super-regens should be sensitive on AM, but are usually poor NBFM detectors. We are about 3km apart and both have V2000 verticals externally mounted. At 70cms these have gain.
Andrew has ordered 5 pairs of 433MHz AM modules (TX as well as RX) for not much more than £2 total from China. This makes each TX/RX pair very inexpensive. It amazes me that they can make these so inexpensively. I hope G6ALB succeeds in this venture. It would be good to be able to communicate with him using a really low cost 70cm transceiver. I expect Andrew will write up his results in an article for a UK magazine later.
It seems entirely possible that a complete 70cm AM transceiver can be made for just a few pounds based on these low cost 433MHz AM modules.
70cms AM
As you may recall, I am quite a fan of super-regen receivers. Nothing as simple is able to match their AM sensitivity, but they tend to have poor selectivity.
With the availability of very low cost 433MHz modules (TX and super-regen RX) designed for AM data, these modules could be ripe for conversion to ultra-simple 70cm AM voice transceivers. One local friend, Andrew G6ALB, is currently carrying out experiments to see if this is feasible. I hope to work him on 70cm AM using such a rig in the months ahead. Sadly, I am still too clumsy to do any building work – very frustrating.
I am wondering if anyone else has tried these modules in such a circuit? With a simple MMIC PA (50 ohms in and out) the power output could be lifted to around 25mW carrier or more. With V2000 verticals this should be good for local ranges. Even at the low milliwatt level from these QRPp modules several km range should be possible with co-linear antennas.
UPDATE 2310z: I wonder what sort of power the TX modules typically produce with linear (speech) modulation rather than 5V square wave data modulation? Presumably much less than their rated power, so an add-on PA may be essential?
