To: The Miami Valley Mesh Alliance
There will be NO MVMA meeting this Saturday, September 10th. We will, instead, attempt a third install of the 2.4 omni node at the DARA Clubhouse (see below). The install party will gather at 8am at the Clubhouse.
The next MVMA meeting will be Saturday, October 8th, at 9:30 am at the Bellbrook Amateur Radio Club (BARC) Clubhouse, located at Sugarcreek Elementary School, 51 South East Street, Bellbrook, OH 45305, Room 1, Lower Level.
The Time Has Come, The Walrus said, to speak of ………… FILTERS! We have had several “learning experiences” using filters over the last two months. A summary of these experiences follows.
- When we installed the 2.4 Omni node at Miami Valley Hospital South, we found that the node receiver was being de-sensed by a paging transmitter, on 940 MHz, running high power (~200 watts) into a high gain antenna located about ten feet from our node antenna. When the pager transmitted, it effectively shut down the node receiver. The problem was corrected by installing a Chinese MicroStrip Band Pass Filter, as shown in the graphic below. The MicroStrip filters are available on eBay for about $12. Tim Procuniar mounted the filter in a box, as shown in the graphic below (or attached); we installed the filter between the Bullet and the L-Com Omni Antenna, and it effectively fixed the de-sense problem.
- When we made the initial node installation at the DARA Clubhouse tower, we found that the 2.4 Node receiver was being de-sensed by the ATV Repeater, located on the same tower. The ATV repeater simulcasts on 421.25 MHz and 1258 Spectrum checks using a HackRF showed that there was no signal interference in the Node’s Channel -2 receive band; rather, the repeater transmitter signal strength appeared to swamp the Node receiver. We made a decision to install a filter on the DARA 2.4 Node.
- Before we could get the filter installed, the DARA Tower took a lightning strike which took out all of our installed equipment.
- We purchased more of the MicroStrip filters, and mounted them in similar fashion, in an all aluminum project box with type N connectors. Both Tim Procuniar, and Tom Holmes help “sweep out” the filters, to determine pass-band frequency limits, and insertion loss. The MicroStrip filters flunked the sweep tests, showing pass-band curves that looked nothing like what we observed previously with the unit installed at MVH-S. We determined that mounting technique was CRITICAL…. keeping exposed coax center conductors SHORT is important.
- At the same time, we acquired a new filter made by a Chinese company called Teletronics International Inc. These 3-pole ceramic filters come pre-packaged with the connectors of your choice. They are sold by RFWirelessUSA.com, located in Maryland, for a cost of $71 each, plus S&H. We also “swept” these filters. They demonstrated excellent pass-band characteristics (which easily covered all of Channel -2 at 10 MHz bandwidth), with an insertion loss of about 3 db.
- We decided to do a performance comparison of the MicroStrip Filter, the Teletronics Filter, and no Filter, at the MVH-S Site. Data recorded at MVH-S is shown in the graphic below (or attached, depending on your email handler). We evaluated each filter by comparing received signal strength from N8NQH and WA8APB nodes, and the strength of the MVH-S signal received AT N8NQH, with and without a pager signal present.
- The bottom line… is that the Teletronics filter significantly outperformed the MicroStrip Both filters blocked the pager signal. Received signal strength measurements were about the same, with and without the pager signal present. However, At MVH-S, we saw a 5 to 8 db increase in receive sensitivity (signal strength), and at N8NQH, we saw an increased signal strength from MVH-S, of about 4 db, using the Teletronics filter, when compared to the MicroStrip unit. We noted that the Teletronics filter also dropped the noise floor, such that the Signal to Noise Ratio with the Teletronics was 28 db, which MATCHED the S/N ratio using no filter at all.
- We decided to suspend use of the MicroStrip filters, in favor of the Teletronics We also concluded that use of a band-pass filter should be standard practice at any location where our 2.4 Nodes will be co-located with other transmission systems (commercial towers, commercial roof-top sites, water tanks, ARRL Field Day, etc.).
- Last Saturday, Phil Verret and I attempted a re-install the 2.4 node with the Teletronics filter, at the top of the DARA tower (170 ft). All equipment was checked out on the ground. Unfortunately, we encountered two problems after install; one of the Cat5e lines opened, and while we had the Node on the air with power over Ethernet, we had no computer control of the node at the bottom of the wire. Worse…. we noted that the Filter N connector between the filter and the antenna failed. The male N connector literally came apart. The install was abandoned, and all equipment returned to the ground. On a positive note, we did successfully install a Microwave receiver link that provides back-up Internet from a P&R Tower to the DARA Clubhouse.
- It appears that a cast flange on the central barrel of the connector separated from the barrel. While we did not intentionally over-tighten the connector, nor impose lateral forces that would cause the connector to fail ….. something must have happened. We have been working with RFWirelessUSA, and they have graciously replaced the filter unit with the bad connector.
- So….. We are ready to work the DARA install again….. and plan to do so this Saturday, September 10th, starting at 8:00am weather permitting. Bruce Kress, KD8YWC and I will climb. If you are interested in participating in ground crew, please contact me by email, or the cell number below.
- We are investigating the possibility of purchasing filters custom tuned (or which we can custom tune) for Channel -2. We would go for a bandwidth of 20 to 30 Such filters would cost approximately $160 (manufacturer tunes) or $80 (we tune). It is not clear that we would realize an overall performance improvement by using the narrower filters, but somehow… I suspect the Filter Saga is not over yet.
L-Com Antennas: L-Com is once again offering their 2.4 GHz 15 dbi Omni HG2415U-PRO antennas for $45, plus $5 shipping, IF purchased in lots of 5 units. (Five antennas will cost $225 plus $25 S&H). If you have interest in participating in a group purchase of these antennas, please contact Moe Riggins, AB8XA, at firstname.lastname@example.org.
That’s all for now.